# `os` — Miscellaneous operating system interfaces

Source code: Lib/os.py

This module provides a portable way of using operating system dependent functionality. If you just want to read or write a file see `open()`, if you want to manipulate paths, see the `os.path` module, and if you want to read all the lines in all the files on the command line see the `fileinput` module. For creating temporary files and directories see the `tempfile` module, and for high-level file and directory handling see the `shutil` module.

Notes on the availability of these functions:

- The design of all built-in operating system dependent modules of Python is such that as long as the same functionality is available, it uses the same interface; for example, the function `os.stat(path)` returns stat information about path in the same format (which happens to have originated with the POSIX interface).

- Extensions peculiar to a particular operating system are also available through the `os` module, but using them is of course a threat to portability.

- All functions accepting path or file names accept both bytes and string objects, and result in an object of the same type, if a path or file name is returned.

- On VxWorks, os.fork, os.execv and os.spawn*p* are not supported.

Note

All functions in this module raise `OSError` (or subclasses thereof) in the case of invalid or inaccessible file names and paths, or other arguments that have the correct type, but are not accepted by the operating system.

exception `os.``error`

  An alias for the built-in `OSError` exception.

`os.``name`

  The name of the operating system dependent module imported. The following names have currently been registered: `'posix'`, `'nt'`, `'java'`.

  See also

  `sys.platform` has a finer granularity. `os.uname()` gives system-dependent version information.

  The `platform` module provides detailed checks for the system’s identity.

## File Names, Command Line Arguments, and Environment Variables

In Python, file names, command line arguments, and environment variables are represented using the string type. On some systems, decoding these strings to and from bytes is necessary before passing them to the operating system. Python uses the file system encoding to perform this conversion (see `sys.getfilesystemencoding()`).

Changed in version 3.1: On some systems, conversion using the file system encoding may fail. In this case, Python uses the surrogateescape encoding error handler, which means that undecodable bytes are replaced by a Unicode character U+DCxx on decoding, and these are again translated to the original byte on encoding.

The file system encoding must guarantee to successfully decode all bytes below 128. If the file system encoding fails to provide this guarantee, API functions may raise UnicodeErrors.

## Process Parameters

These functions and data items provide information and operate on the current process and user.

`os.``ctermid`()

  Return the filename corresponding to the controlling terminal of the process.

  Availability: Unix.

`os.``environ`

  A mapping object where keys and values are strings that represent the process environment. For example, `environ['HOME']` is the pathname of your home directory (on some platforms), and is equivalent to `getenv("HOME")` in C.

  This mapping is captured the first time the `os` module is imported, typically during Python startup as part of processing `site.py`. Changes to the environment made after this time are not reflected in `os.environ`, except for changes made by modifying `os.environ` directly.

  This mapping may be used to modify the environment as well as query the environment. `putenv()` will be called automatically when the mapping is modified.

  On Unix, keys and values use `sys.getfilesystemencoding()` and `'surrogateescape'` error handler. Use `environb` if you would like to use a different encoding.

  Note

  Calling `putenv()` directly does not change `os.environ`, so it’s better to modify `os.environ`.

  Note

  On some platforms, including FreeBSD and macOS, setting `environ` may cause memory leaks. Refer to the system documentation for `putenv()`.

  You can delete items in this mapping to unset environment variables. `unsetenv()` will be called automatically when an item is deleted from `os.environ`, and when one of the `pop()` or `clear()` methods is called.

  Changed in version 3.9: Updated to support PEP 584’s merge (`|`) and update (`|=`) operators.

`os.``environb`

  Bytes version of `environ`: a mapping object where both keys and values are `bytes` objects representing the process environment. `environ` and `environb` are synchronized (modifying `environb` updates `environ`, and vice versa).

  `environb` is only available if `supports_bytes_environ` is `True`.

  New in version 3.2.

  Changed in version 3.9: Updated to support PEP 584’s merge (`|`) and update (`|=`) operators.

`os.``chdir`(path)
`os.``fchdir`(fd)
`os.``getcwd`()

  These functions are described in Files and Directories.

`os.``fsencode`(filename)

  Encode path-like filename to the filesystem encoding with `'surrogateescape'` error handler, or `'strict'` on Windows; return `bytes` unchanged.

  `fsdecode()` is the reverse function.

  New in version 3.2.

  Changed in version 3.6: Support added to accept objects implementing the `os.PathLike` interface.

`os.``fsdecode`(filename)

  Decode the path-like filename from the filesystem encoding with `'surrogateescape'` error handler, or `'strict'` on Windows; return `str` unchanged.

  `fsencode()` is the reverse function.

  New in version 3.2.

  Changed in version 3.6: Support added to accept objects implementing the `os.PathLike` interface.

`os.``fspath`(path)

  Return the file system representation of the path.

  If `str` or `bytes` is passed in, it is returned unchanged. Otherwise `__fspath__()` is called and its value is returned as long as it is a `str` or `bytes` object. In all other cases, `TypeError` is raised.

  New in version 3.6.

class `os.``PathLike`

  An abstract base class for objects representing a file system path, e.g. `pathlib.PurePath`.

  New in version 3.6.

  abstractmethod `__fspath__`()

    Return the file system path representation of the object.

    The method should only return a `str` or `bytes` object, with the preference being for `str`.

`os.``getenv`(key, default=None)

  Return the value of the environment variable key if it exists, or default if it doesn’t. key, default and the result are str. Note that since `getenv()` uses `os.environ`, the mapping of `getenv()` is similarly also captured on import, and the function may not reflect future environment changes.

  On Unix, keys and values are decoded with `sys.getfilesystemencoding()` and `'surrogateescape'` error handler. Use `os.getenvb()` if you would like to use a different encoding.

  Availability: most flavors of Unix, Windows.

`os.``getenvb`(key, default=None)

  Return the value of the environment variable key if it exists, or default if it doesn’t. key, default and the result are bytes. Note that since `getenvb()` uses `os.environb`, the mapping of `getenvb()` is similarly also captured on import, and the function may not reflect future environment changes.

  `getenvb()` is only available if `supports_bytes_environ` is `True`.

  Availability: most flavors of Unix.

  New in version 3.2.

`os.``get_exec_path`(env=None)

  Returns the list of directories that will be searched for a named executable, similar to a shell, when launching a process. env, when specified, should be an environment variable dictionary to lookup the PATH in. By default, when env is `None`, `environ` is used.

  New in version 3.2.

`os.``getegid`()

  Return the effective group id of the current process. This corresponds to the “set id” bit on the file being executed in the current process.

  Availability: Unix.

`os.``geteuid`()

  Return the current process’s effective user id.

  Availability: Unix.

`os.``getgid`()

  Return the real group id of the current process.

  Availability: Unix.

`os.``getgrouplist`(user, group)

  Return list of group ids that user belongs to. If group is not in the list, it is included; typically, group is specified as the group ID field from the password record for user, because that group ID will otherwise be potentially omitted.

  Availability: Unix.

  New in version 3.3.

`os.``getgroups`()

  Return list of supplemental group ids associated with the current process.

  Availability: Unix.

  Note

  On macOS, `getgroups()` behavior differs somewhat from other Unix platforms. If the Python interpreter was built with a deployment target of `10.5` or earlier, `getgroups()` returns the list of effective group ids associated with the current user process; this list is limited to a system-defined number of entries, typically 16, and may be modified by calls to `setgroups()` if suitably privileged. If built with a deployment target greater than `10.5`, `getgroups()` returns the current group access list for the user associated with the effective user id of the process; the group access list may change over the lifetime of the process, it is not affected by calls to `setgroups()`, and its length is not limited to 16. The deployment target value, `MACOSX_DEPLOYMENT_TARGET`, can be obtained with `sysconfig.get_config_var()`.

`os.``getlogin`()

  Return the name of the user logged in on the controlling terminal of the process. For most purposes, it is more useful to use `getpass.getuser()` since the latter checks the environment variables `LOGNAME` or `USERNAME` to find out who the user is, and falls back to `pwd.getpwuid(os.getuid())[0]` to get the login name of the current real user id.

  Availability: Unix, Windows.

`os.``getpgid`(pid)

  Return the process group id of the process with process id pid. If pid is 0, the process group id of the current process is returned.

  Availability: Unix.

`os.``getpgrp`()

  Return the id of the current process group.

  Availability: Unix.

`os.``getpid`()

  Return the current process id.

`os.``getppid`()

  Return the parent’s process id. When the parent process has exited, on Unix the id returned is the one of the init process (1), on Windows it is still the same id, which may be already reused by another process.

  Availability: Unix, Windows.

  Changed in version 3.2: Added support for Windows.

`os.``getpriority`(which, who)

  Get program scheduling priority. The value which is one of `PRIO_PROCESS`, `PRIO_PGRP`, or `PRIO_USER`, and who is interpreted relative to which (a process identifier for `PRIO_PROCESS`, process group identifier for `PRIO_PGRP`, and a user ID for `PRIO_USER`). A zero value for who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process.

  Availability: Unix.

  New in version 3.3.

`os.``PRIO_PROCESS`
`os.``PRIO_PGRP`
`os.``PRIO_USER`

  Parameters for the `getpriority()` and `setpriority()` functions.

  Availability: Unix.

  New in version 3.3.

`os.``getresuid`()

  Return a tuple (ruid, euid, suid) denoting the current process’s real, effective, and saved user ids.

  Availability: Unix.

  New in version 3.2.

`os.``getresgid`()

  Return a tuple (rgid, egid, sgid) denoting the current process’s real, effective, and saved group ids.

  Availability: Unix.

  New in version 3.2.

`os.``getuid`()

  Return the current process’s real user id.

  Availability: Unix.

`os.``initgroups`(username, gid)

  Call the system initgroups() to initialize the group access list with all of the groups of which the specified username is a member, plus the specified group id.

  Availability: Unix.

  New in version 3.2.

`os.``putenv`(key, value)

  Set the environment variable named key to the string value. Such changes to the environment affect subprocesses started with `os.system()`, `popen()` or `fork()` and `execv()`.

  Assignments to items in `os.environ` are automatically translated into corresponding calls to `putenv()`; however, calls to `putenv()` don’t update `os.environ`, so it is actually preferable to assign to items of `os.environ`. This also applies to `getenv()` and `getenvb()`, which respectively use `os.environ` and `os.environb` in their implementations.

  Note

  On some platforms, including FreeBSD and macOS, setting `environ` may cause memory leaks. Refer to the system documentation for `putenv()`.

  Raises an auditing event `os.putenv` with arguments `key`, `value`.

  Changed in version 3.9: The function is now always available.

`os.``setegid`(egid)

  Set the current process’s effective group id.

  Availability: Unix.

`os.``seteuid`(euid)

  Set the current process’s effective user id.

  Availability: Unix.

`os.``setgid`(gid)

  Set the current process’ group id.

  Availability: Unix.

`os.``setgroups`(groups)

  Set the list of supplemental group ids associated with the current process to groups. groups must be a sequence, and each element must be an integer identifying a group. This operation is typically available only to the superuser.

  Availability: Unix.

  Note

  On macOS, the length of groups may not exceed the system-defined maximum number of effective group ids, typically 16. See the documentation for `getgroups()` for cases where it may not return the same group list set by calling setgroups().

`os.``setpgrp`()

  Call the system call `setpgrp()` or `setpgrp(0, 0)` depending on which version is implemented (if any). See the Unix manual for the semantics.

  Availability: Unix.

`os.``setpgid`(pid, pgrp)

  Call the system call `setpgid()` to set the process group id of the process with id pid to the process group with id pgrp. See the Unix manual for the semantics.

  Availability: Unix.

`os.``setpriority`(which, who, priority)

  Set program scheduling priority. The value which is one of `PRIO_PROCESS`, `PRIO_PGRP`, or `PRIO_USER`, and who is interpreted relative to which (a process identifier for `PRIO_PROCESS`, process group identifier for `PRIO_PGRP`, and a user ID for `PRIO_USER`). A zero value for who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process. priority is a value in the range -20 to 19. The default priority is 0; lower priorities cause more favorable scheduling.

  Availability: Unix.

  New in version 3.3.

`os.``setregid`(rgid, egid)

  Set the current process’s real and effective group ids.

  Availability: Unix.

`os.``setresgid`(rgid, egid, sgid)

  Set the current process’s real, effective, and saved group ids.

  Availability: Unix.

  New in version 3.2.

`os.``setresuid`(ruid, euid, suid)

  Set the current process’s real, effective, and saved user ids.

  Availability: Unix.

  New in version 3.2.

`os.``setreuid`(ruid, euid)

  Set the current process’s real and effective user ids.

  Availability: Unix.

`os.``getsid`(pid)

  Call the system call `getsid()`. See the Unix manual for the semantics.

  Availability: Unix.

`os.``setsid`()

  Call the system call `setsid()`. See the Unix manual for the semantics.

  Availability: Unix.

`os.``setuid`(uid)

  Set the current process’s user id.

  Availability: Unix.

`os.``strerror`(code)

  Return the error message corresponding to the error code in code. On platforms where `strerror()` returns `NULL` when given an unknown error number, `ValueError` is raised.

`os.``supports_bytes_environ`

  `True` if the native OS type of the environment is bytes (eg. `False` on Windows).

  New in version 3.2.

`os.``umask`(mask)

  Set the current numeric umask and return the previous umask.

`os.``uname`()

  Returns information identifying the current operating system. The return value is an object with five attributes:

  - `sysname` - operating system name

  - `nodename` - name of machine on network (implementation-defined)

  - `release` - operating system release

  - `version` - operating system version

  - `machine` - hardware identifier

  For backwards compatibility, this object is also iterable, behaving like a five-tuple containing `sysname`, `nodename`, `release`, `version`, and `machine` in that order.

  Some systems truncate `nodename` to 8 characters or to the leading component; a better way to get the hostname is `socket.gethostname()` or even `socket.gethostbyaddr(socket.gethostname())`.

  Availability: recent flavors of Unix.

  Changed in version 3.3: Return type changed from a tuple to a tuple-like object with named attributes.

`os.``unsetenv`(key)

  Unset (delete) the environment variable named key. Such changes to the environment affect subprocesses started with `os.system()`, `popen()` or `fork()` and `execv()`.

  Deletion of items in `os.environ` is automatically translated into a corresponding call to `unsetenv()`; however, calls to `unsetenv()` don’t update `os.environ`, so it is actually preferable to delete items of `os.environ`.

  Raises an auditing event `os.unsetenv` with argument `key`.

  Changed in version 3.9: The function is now always available and is also available on Windows.

## File Object Creation

These functions create new file objects. (See also `open()` for opening file descriptors.)

`os.``fdopen`(fd, *args, **kwargs)

  Return an open file object connected to the file descriptor fd. This is an alias of the `open()` built-in function and accepts the same arguments. The only difference is that the first argument of `fdopen()` must always be an integer.

## File Descriptor Operations

These functions operate on I/O streams referenced using file descriptors.

File descriptors are small integers corresponding to a file that has been opened by the current process. For example, standard input is usually file descriptor 0, standard output is 1, and standard error is 2. Further files opened by a process will then be assigned 3, 4, 5, and so forth. The name “file descriptor” is slightly deceptive; on Unix platforms, sockets and pipes are also referenced by file descriptors.

The `fileno()` method can be used to obtain the file descriptor associated with a file object when required. Note that using the file descriptor directly will bypass the file object methods, ignoring aspects such as internal buffering of data.

`os.``close`(fd)

  Close file descriptor fd.

  Note

  This function is intended for low-level I/O and must be applied to a file descriptor as returned by `os.open()` or `pipe()`. To close a “file object” returned by the built-in function `open()` or by `popen()` or `fdopen()`, use its `close()` method.

`os.``closerange`(fd_low, fd_high)

  Close all file descriptors from fd_low (inclusive) to fd_high (exclusive), ignoring errors. Equivalent to (but much faster than):

      for fd in range(fd_low, fd_high):
          try:
              os.close(fd)
          except OSError:
              pass

`os.``copy_file_range`(src, dst, count, offset_src=None, offset_dst=None)

  Copy count bytes from file descriptor src, starting from offset offset_src, to file descriptor dst, starting from offset offset_dst. If offset_src is None, then src is read from the current position; respectively for offset_dst. The files pointed by src and dst must reside in the same filesystem, otherwise an `OSError` is raised with `errno` set to `errno.EXDEV`.

  This copy is done without the additional cost of transferring data from the kernel to user space and then back into the kernel. Additionally, some filesystems could implement extra optimizations. The copy is done as if both files are opened as binary.

  The return value is the amount of bytes copied. This could be less than the amount requested.

  Availability: Linux kernel >= 4.5 or glibc >= 2.27.

  New in version 3.8.

`os.``device_encoding`(fd)

  Return a string describing the encoding of the device associated with fd if it is connected to a terminal; else return `None`.

`os.``dup`(fd)

  Return a duplicate of file descriptor fd. The new file descriptor is non-inheritable.

  On Windows, when duplicating a standard stream (0: stdin, 1: stdout, 2: stderr), the new file descriptor is inheritable.

  Changed in version 3.4: The new file descriptor is now non-inheritable.

`os.``dup2`(fd, fd2, inheritable=True)

  Duplicate file descriptor fd to fd2, closing the latter first if necessary. Return fd2. The new file descriptor is inheritable by default or non-inheritable if inheritable is `False`.

  Changed in version 3.4: Add the optional inheritable parameter.

  Changed in version 3.7: Return fd2 on success. Previously, `None` was always returned.

`os.``fchmod`(fd, mode)

  Change the mode of the file given by fd to the numeric mode. See the docs for `chmod()` for possible values of mode. As of Python 3.3, this is equivalent to `os.chmod(fd, mode)`.

  Raises an auditing event `os.chmod` with arguments `path`, `mode`, `dir_fd`.

  Availability: Unix.

`os.``fchown`(fd, uid, gid)

  Change the owner and group id of the file given by fd to the numeric uid and gid. To leave one of the ids unchanged, set it to -1. See `chown()`. As of Python 3.3, this is equivalent to `os.chown(fd, uid, gid)`.

  Raises an auditing event `os.chown` with arguments `path`, `uid`, `gid`, `dir_fd`.

  Availability: Unix.

`os.``fdatasync`(fd)

  Force write of file with filedescriptor fd to disk. Does not force update of metadata.

  Availability: Unix.

  Note

  This function is not available on MacOS.

`os.``fpathconf`(fd, name)

  Return system configuration information relevant to an open file. name specifies the configuration value to retrieve; it may be a string which is the name of a defined system value; these names are specified in a number of standards (POSIX.1, Unix 95, Unix 98, and others). Some platforms define additional names as well. The names known to the host operating system are given in the `pathconf_names` dictionary. For configuration variables not included in that mapping, passing an integer for name is also accepted.

  If name is a string and is not known, `ValueError` is raised. If a specific value for name is not supported by the host system, even if it is included in `pathconf_names`, an `OSError` is raised with `errno.EINVAL` for the error number.

  As of Python 3.3, this is equivalent to `os.pathconf(fd, name)`.

  Availability: Unix.

`os.``fstat`(fd)

  Get the status of the file descriptor fd. Return a `stat_result` object.

  As of Python 3.3, this is equivalent to `os.stat(fd)`.

  See also

  The `stat()` function.

`os.``fstatvfs`(fd)

  Return information about the filesystem containing the file associated with file descriptor fd, like `statvfs()`. As of Python 3.3, this is equivalent to `os.statvfs(fd)`.

  Availability: Unix.

`os.``fsync`(fd)

  Force write of file with filedescriptor fd to disk. On Unix, this calls the native `fsync()` function; on Windows, the MS `_commit()` function.

  If you’re starting with a buffered Python file object f, first do `f.flush()`, and then do `os.fsync(f.fileno())`, to ensure that all internal buffers associated with f are written to disk.

  Availability: Unix, Windows.

`os.``ftruncate`(fd, length)

  Truncate the file corresponding to file descriptor fd, so that it is at most length bytes in size. As of Python 3.3, this is equivalent to `os.truncate(fd, length)`.

  Raises an auditing event `os.truncate` with arguments `fd`, `length`.

  Availability: Unix, Windows.

  Changed in version 3.5: Added support for Windows

`os.``get_blocking`(fd)

  Get the blocking mode of the file descriptor: `False` if the `O_NONBLOCK` flag is set, `True` if the flag is cleared.

  See also `set_blocking()` and `socket.socket.setblocking()`.

  Availability: Unix.

  New in version 3.5.

`os.``isatty`(fd)

  Return `True` if the file descriptor fd is open and connected to a tty(-like) device, else `False`.

`os.``lockf`(fd, cmd, len)

  Apply, test or remove a POSIX lock on an open file descriptor. fd is an open file descriptor. cmd specifies the command to use - one of `F_LOCK`, `F_TLOCK`, `F_ULOCK` or `F_TEST`. len specifies the section of the file to lock.

  Raises an auditing event `os.lockf` with arguments `fd`, `cmd`, `len`.

  Availability: Unix.

  New in version 3.3.

`os.``F_LOCK`
`os.``F_TLOCK`
`os.``F_ULOCK`
`os.``F_TEST`

  Flags that specify what action `lockf()` will take.

  Availability: Unix.

  New in version 3.3.

`os.``lseek`(fd, pos, how)

  Set the current position of file descriptor fd to position pos, modified by how: `SEEK_SET` or `0` to set the position relative to the beginning of the file; `SEEK_CUR` or `1` to set it relative to the current position; `SEEK_END` or `2` to set it relative to the end of the file. Return the new cursor position in bytes, starting from the beginning.

`os.``SEEK_SET`
`os.``SEEK_CUR`
`os.``SEEK_END`

  Parameters to the `lseek()` function. Their values are 0, 1, and 2, respectively.

  New in version 3.3: Some operating systems could support additional values, like `os.SEEK_HOLE` or `os.SEEK_DATA`.

`os.``open`(path, flags, mode=0o777, *, dir_fd=None)

  Open the file path and set various flags according to flags and possibly its mode according to mode. When computing mode, the current umask value is first masked out. Return the file descriptor for the newly opened file. The new file descriptor is non-inheritable.

  For a description of the flag and mode values, see the C run-time documentation; flag constants (like `O_RDONLY` and `O_WRONLY`) are defined in the `os` module. In particular, on Windows adding `O_BINARY` is needed to open files in binary mode.

  This function can support paths relative to directory descriptors with the dir_fd parameter.

  Raises an auditing event `open` with arguments `path`, `mode`, `flags`.

  Changed in version 3.4: The new file descriptor is now non-inheritable.

  Note

  This function is intended for low-level I/O. For normal usage, use the built-in function `open()`, which returns a file object with `read()` and `write()` methods (and many more). To wrap a file descriptor in a file object, use `fdopen()`.

  New in version 3.3: The dir_fd argument.

  Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the function now retries the system call instead of raising an `InterruptedError` exception (see PEP 475 for the rationale).

  Changed in version 3.6: Accepts a path-like object.

The following constants are options for the flags parameter to the `open()` function. They can be combined using the bitwise OR operator `|`. Some of them are not available on all platforms. For descriptions of their availability and use, consult the open(2) manual page on Unix or the MSDN on Windows.

`os.``O_RDONLY`
`os.``O_WRONLY`
`os.``O_RDWR`
`os.``O_APPEND`
`os.``O_CREAT`
`os.``O_EXCL`
`os.``O_TRUNC`

  The above constants are available on Unix and Windows.

`os.``O_DSYNC`
`os.``O_RSYNC`
`os.``O_SYNC`
`os.``O_NDELAY`
`os.``O_NONBLOCK`
`os.``O_NOCTTY`
`os.``O_CLOEXEC`

  The above constants are only available on Unix.

  Changed in version 3.3: Add `O_CLOEXEC` constant.

`os.``O_BINARY`
`os.``O_NOINHERIT`
`os.``O_SHORT_LIVED`
`os.``O_TEMPORARY`
`os.``O_RANDOM`
`os.``O_SEQUENTIAL`
`os.``O_TEXT`

  The above constants are only available on Windows.

`os.``O_ASYNC`
`os.``O_DIRECT`
`os.``O_DIRECTORY`
`os.``O_NOFOLLOW`
`os.``O_NOATIME`
`os.``O_PATH`
`os.``O_TMPFILE`
`os.``O_SHLOCK`
`os.``O_EXLOCK`

  The above constants are extensions and not present if they are not defined by the C library.

  Changed in version 3.4: Add `O_PATH` on systems that support it. Add `O_TMPFILE`, only available on Linux Kernel 3.11 or newer.

`os.``openpty`()

  Open a new pseudo-terminal pair. Return a pair of file descriptors `(master, slave)` for the pty and the tty, respectively. The new file descriptors are non-inheritable. For a (slightly) more portable approach, use the `pty` module.

  Availability: some flavors of Unix.

  Changed in version 3.4: The new file descriptors are now non-inheritable.

`os.``pipe`()

  Create a pipe. Return a pair of file descriptors `(r, w)` usable for reading and writing, respectively. The new file descriptor is non-inheritable.

  Availability: Unix, Windows.

  Changed in version 3.4: The new file descriptors are now non-inheritable.

`os.``pipe2`(flags)

  Create a pipe with flags set atomically. flags can be constructed by ORing together one or more of these values: `O_NONBLOCK`, `O_CLOEXEC`. Return a pair of file descriptors `(r, w)` usable for reading and writing, respectively.

  Availability: some flavors of Unix.

  New in version 3.3.

`os.``posix_fallocate`(fd, offset, len)

  Ensures that enough disk space is allocated for the file specified by fd starting from offset and continuing for len bytes.

  Availability: Unix.

  New in version 3.3.

`os.``posix_fadvise`(fd, offset, len, advice)

  Announces an intention to access data in a specific pattern thus allowing the kernel to make optimizations. The advice applies to the region of the file specified by fd starting at offset and continuing for len bytes. advice is one of `POSIX_FADV_NORMAL`, `POSIX_FADV_SEQUENTIAL`, `POSIX_FADV_RANDOM`, `POSIX_FADV_NOREUSE`, `POSIX_FADV_WILLNEED` or `POSIX_FADV_DONTNEED`.

  Availability: Unix.

  New in version 3.3.

`os.``POSIX_FADV_NORMAL`
`os.``POSIX_FADV_SEQUENTIAL`
`os.``POSIX_FADV_RANDOM`
`os.``POSIX_FADV_NOREUSE`
`os.``POSIX_FADV_WILLNEED`
`os.``POSIX_FADV_DONTNEED`

  Flags that can be used in advice in `posix_fadvise()` that specify the access pattern that is likely to be used.

  Availability: Unix.

  New in version 3.3.

`os.``pread`(fd, n, offset)

  Read at most n bytes from file descriptor fd at a position of offset, leaving the file offset unchanged.

  Return a bytestring containing the bytes read. If the end of the file referred to by fd has been reached, an empty bytes object is returned.

  Availability: Unix.

  New in version 3.3.

`os.``preadv`(fd, buffers, offset, flags=0)

  Read from a file descriptor fd at a position of offset into mutable bytes-like objects buffers, leaving the file offset unchanged. Transfer data into each buffer until it is full and then move on to the next buffer in the sequence to hold the rest of the data.

  The flags argument contains a bitwise OR of zero or more of the following flags:

  - `RWF_HIPRI`

  - `RWF_NOWAIT`

  Return the total number of bytes actually read which can be less than the total capacity of all the objects.

  The operating system may set a limit (`sysconf()` value `'SC_IOV_MAX'`) on the number of buffers that can be used.

  Combine the functionality of `os.readv()` and `os.pread()`.

  Availability: Linux 2.6.30 and newer, FreeBSD 6.0 and newer, OpenBSD 2.7 and newer, AIX 7.1 and newer. Using flags requires Linux 4.6 or newer.

  New in version 3.7.

`os.``RWF_NOWAIT`

  Do not wait for data which is not immediately available. If this flag is specified, the system call will return instantly if it would have to read data from the backing storage or wait for a lock.

  If some data was successfully read, it will return the number of bytes read. If no bytes were read, it will return `-1` and set errno to `errno.EAGAIN`.

  Availability: Linux 4.14 and newer.

  New in version 3.7.

`os.``RWF_HIPRI`

  High priority read/write. Allows block-based filesystems to use polling of the device, which provides lower latency, but may use additional resources.

  Currently, on Linux, this feature is usable only on a file descriptor opened using the `O_DIRECT` flag.

  Availability: Linux 4.6 and newer.

  New in version 3.7.

`os.``pwrite`(fd, str, offset)

  Write the bytestring in str to file descriptor fd at position of offset, leaving the file offset unchanged.

  Return the number of bytes actually written.

  Availability: Unix.

  New in version 3.3.

`os.``pwritev`(fd, buffers, offset, flags=0)

  Write the buffers contents to file descriptor fd at a offset offset, leaving the file offset unchanged. buffers must be a sequence of bytes-like objects. Buffers are processed in array order. Entire contents of the first buffer is written before proceeding to the second, and so on.

  The flags argument contains a bitwise OR of zero or more of the following flags:

  - `RWF_DSYNC`

  - `RWF_SYNC`

  Return the total number of bytes actually written.

  The operating system may set a limit (`sysconf()` value `'SC_IOV_MAX'`) on the number of buffers that can be used.

  Combine the functionality of `os.writev()` and `os.pwrite()`.

  Availability: Linux 2.6.30 and newer, FreeBSD 6.0 and newer, OpenBSD 2.7 and newer, AIX 7.1 and newer. Using flags requires Linux 4.7 or newer.

  New in version 3.7.

`os.``RWF_DSYNC`

  Provide a per-write equivalent of the `O_DSYNC` `open(2)` flag. This flag effect applies only to the data range written by the system call.

  Availability: Linux 4.7 and newer.

  New in version 3.7.

`os.``RWF_SYNC`

  Provide a per-write equivalent of the `O_SYNC` `open(2)` flag. This flag effect applies only to the data range written by the system call.

  Availability: Linux 4.7 and newer.

  New in version 3.7.

`os.``read`(fd, n)

  Read at most n bytes from file descriptor fd.

  Return a bytestring containing the bytes read. If the end of the file referred to by fd has been reached, an empty bytes object is returned.

  Note

  This function is intended for low-level I/O and must be applied to a file descriptor as returned by `os.open()` or `pipe()`. To read a “file object” returned by the built-in function `open()` or by `popen()` or `fdopen()`, or `sys.stdin`, use its `read()` or `readline()` methods.

  Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the function now retries the system call instead of raising an `InterruptedError` exception (see PEP 475 for the rationale).

`os.``sendfile`(out_fd, in_fd, offset, count)
`os.``sendfile`(out_fd, in_fd, offset, count, headers=(), trailers=(), flags=0)

  Copy count bytes from file descriptor in_fd to file descriptor out_fd starting at offset. Return the number of bytes sent. When EOF is reached return `0`.

  The first function notation is supported by all platforms that define `sendfile()`.

  On Linux, if offset is given as `None`, the bytes are read from the current position of in_fd and the position of in_fd is updated.

  The second case may be used on macOS and FreeBSD where headers and trailers are arbitrary sequences of buffers that are written before and after the data from in_fd is written. It returns the same as the first case.

  On macOS and FreeBSD, a value of `0` for count specifies to send until the end of in_fd is reached.

  All platforms support sockets as out_fd file descriptor, and some platforms allow other types (e.g. regular file, pipe) as well.

  Cross-platform applications should not use headers, trailers and flags arguments.

  Availability: Unix.

  Note

  For a higher-level wrapper of `sendfile()`, see `socket.socket.sendfile()`.

  New in version 3.3.

  Changed in version 3.9: Parameters out and in was renamed to out_fd and in_fd.

`os.``set_blocking`(fd, blocking)

  Set the blocking mode of the specified file descriptor. Set the `O_NONBLOCK` flag if blocking is `False`, clear the flag otherwise.

  See also `get_blocking()` and `socket.socket.setblocking()`.

  Availability: Unix.

  New in version 3.5.

`os.``SF_NODISKIO`
`os.``SF_MNOWAIT`
`os.``SF_SYNC`

  Parameters to the `sendfile()` function, if the implementation supports them.

  Availability: Unix.

  New in version 3.3.

`os.``readv`(fd, buffers)

  Read from a file descriptor fd into a number of mutable bytes-like objects buffers. Transfer data into each buffer until it is full and then move on to the next buffer in the sequence to hold the rest of the data.

  Return the total number of bytes actually read which can be less than the total capacity of all the objects.

  The operating system may set a limit (`sysconf()` value `'SC_IOV_MAX'`) on the number of buffers that can be used.

  Availability: Unix.

  New in version 3.3.

`os.``tcgetpgrp`(fd)

  Return the process group associated with the terminal given by fd (an open file descriptor as returned by `os.open()`).

  Availability: Unix.

`os.``tcsetpgrp`(fd, pg)

  Set the process group associated with the terminal given by fd (an open file descriptor as returned by `os.open()`) to pg.

  Availability: Unix.

`os.``ttyname`(fd)

  Return a string which specifies the terminal device associated with file descriptor fd. If fd is not associated with a terminal device, an exception is raised.

  Availability: Unix.

`os.``write`(fd, str)

  Write the bytestring in str to file descriptor fd.

  Return the number of bytes actually written.

  Note

  This function is intended for low-level I/O and must be applied to a file descriptor as returned by `os.open()` or `pipe()`. To write a “file object” returned by the built-in function `open()` or by `popen()` or `fdopen()`, or `sys.stdout` or `sys.stderr`, use its `write()` method.

  Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the function now retries the system call instead of raising an `InterruptedError` exception (see PEP 475 for the rationale).

`os.``writev`(fd, buffers)

  Write the contents of buffers to file descriptor fd. buffers must be a sequence of bytes-like objects. Buffers are processed in array order. Entire contents of the first buffer is written before proceeding to the second, and so on.

  Returns the total number of bytes actually written.

  The operating system may set a limit (`sysconf()` value `'SC_IOV_MAX'`) on the number of buffers that can be used.

  Availability: Unix.

  New in version 3.3.

### Querying the size of a terminal

New in version 3.3.

`os.``get_terminal_size`(fd=STDOUT_FILENO)

  Return the size of the terminal window as `(columns, lines)`, tuple of type `terminal_size`.

  The optional argument `fd` (default `STDOUT_FILENO`, or standard output) specifies which file descriptor should be queried.

  If the file descriptor is not connected to a terminal, an `OSError` is raised.

  `shutil.get_terminal_size()` is the high-level function which should normally be used, `os.get_terminal_size` is the low-level implementation.

  Availability: Unix, Windows.

class `os.``terminal_size`

  A subclass of tuple, holding `(columns, lines)` of the terminal window size.

  `columns`

    Width of the terminal window in characters.

  `lines`

    Height of the terminal window in characters.

### Inheritance of File Descriptors

New in version 3.4.

A file descriptor has an “inheritable” flag which indicates if the file descriptor can be inherited by child processes. Since Python 3.4, file descriptors created by Python are non-inheritable by default.

On UNIX, non-inheritable file descriptors are closed in child processes at the execution of a new program, other file descriptors are inherited.

On Windows, non-inheritable handles and file descriptors are closed in child processes, except for standard streams (file descriptors 0, 1 and 2: stdin, stdout and stderr), which are always inherited. Using `spawn*` functions, all inheritable handles and all inheritable file descriptors are inherited. Using the `subprocess` module, all file descriptors except standard streams are closed, and inheritable handles are only inherited if the close_fds parameter is `False`.

`os.``get_inheritable`(fd)

  Get the “inheritable” flag of the specified file descriptor (a boolean).

`os.``set_inheritable`(fd, inheritable)

  Set the “inheritable” flag of the specified file descriptor.

`os.``get_handle_inheritable`(handle)

  Get the “inheritable” flag of the specified handle (a boolean).

  Availability: Windows.

`os.``set_handle_inheritable`(handle, inheritable)

  Set the “inheritable” flag of the specified handle.

  Availability: Windows.

## Files and Directories

On some Unix platforms, many of these functions support one or more of these features:

- specifying a file descriptor: Normally the path argument provided to functions in the `os` module must be a string specifying a file path. However, some functions now alternatively accept an open file descriptor for their path argument. The function will then operate on the file referred to by the descriptor. (For POSIX systems, Python will call the variant of the function prefixed with `f` (e.g. call `fchdir` instead of `chdir`).)

  You can check whether or not path can be specified as a file descriptor for a particular function on your platform using `os.supports_fd`. If this functionality is unavailable, using it will raise a `NotImplementedError`.

  If the function also supports dir_fd or follow_symlinks arguments, it’s an error to specify one of those when supplying path as a file descriptor.

- paths relative to directory descriptors: If dir_fd is not `None`, it should be a file descriptor referring to a directory, and the path to operate on should be relative; path will then be relative to that directory. If the path is absolute, dir_fd is ignored. (For POSIX systems, Python will call the variant of the function with an `at` suffix and possibly prefixed with `f` (e.g. call `faccessat` instead of `access`).

  You can check whether or not dir_fd is supported for a particular function on your platform using `os.supports_dir_fd`. If it’s unavailable, using it will raise a `NotImplementedError`.

- not following symlinks: If follow_symlinks is `False`, and the last element of the path to operate on is a symbolic link, the function will operate on the symbolic link itself rather than the file pointed to by the link. (For POSIX systems, Python will call the `l...` variant of the function.)

  You can check whether or not follow_symlinks is supported for a particular function on your platform using `os.supports_follow_symlinks`. If it’s unavailable, using it will raise a `NotImplementedError`.

`os.``access`(path, mode, *, dir_fd=None, effective_ids=False, follow_symlinks=True)

  Use the real uid/gid to test for access to path. Note that most operations will use the effective uid/gid, therefore this routine can be used in a suid/sgid environment to test if the invoking user has the specified access to path. mode should be `F_OK` to test the existence of path, or it can be the inclusive OR of one or more of `R_OK`, `W_OK`, and `X_OK` to test permissions. Return `True` if access is allowed, `False` if not. See the Unix man page access(2) for more information.

  This function can support specifying paths relative to directory descriptors and not following symlinks.

  If effective_ids is `True`, `access()` will perform its access checks using the effective uid/gid instead of the real uid/gid. effective_ids may not be supported on your platform; you can check whether or not it is available using `os.supports_effective_ids`. If it is unavailable, using it will raise a `NotImplementedError`.

  Note

  Using `access()` to check if a user is authorized to e.g. open a file before actually doing so using `open()` creates a security hole, because the user might exploit the short time interval between checking and opening the file to manipulate it. It’s preferable to use EAFP techniques. For example:

      if os.access("myfile", os.R_OK):
          with open("myfile") as fp:
              return fp.read()
      return "some default data"

  is better written as:

      try:
          fp = open("myfile")
      except PermissionError:
          return "some default data"
      else:
          with fp:
              return fp.read()

  Note

  I/O operations may fail even when `access()` indicates that they would succeed, particularly for operations on network filesystems which may have permissions semantics beyond the usual POSIX permission-bit model.

  Changed in version 3.3: Added the dir_fd, effective_ids, and follow_symlinks parameters.

  Changed in version 3.6: Accepts a path-like object.

`os.``F_OK`
`os.``R_OK`
`os.``W_OK`
`os.``X_OK`

  Values to pass as the mode parameter of `access()` to test the existence, readability, writability and executability of path, respectively.

`os.``chdir`(path)

  Change the current working directory to path.

  This function can support specifying a file descriptor. The descriptor must refer to an opened directory, not an open file.

  This function can raise `OSError` and subclasses such as `FileNotFoundError`, `PermissionError`, and `NotADirectoryError`.

  Raises an auditing event `os.chdir` with argument `path`.

  New in version 3.3: Added support for specifying path as a file descriptor on some platforms.

  Changed in version 3.6: Accepts a path-like object.

`os.``chflags`(path, flags, *, follow_symlinks=True)

  Set the flags of path to the numeric flags. flags may take a combination (bitwise OR) of the following values (as defined in the `stat` module):

  - `stat.UF_NODUMP`

  - `stat.UF_IMMUTABLE`

  - `stat.UF_APPEND`

  - `stat.UF_OPAQUE`

  - `stat.UF_NOUNLINK`

  - `stat.UF_COMPRESSED`

  - `stat.UF_HIDDEN`

  - `stat.SF_ARCHIVED`

  - `stat.SF_IMMUTABLE`

  - `stat.SF_APPEND`

  - `stat.SF_NOUNLINK`

  - `stat.SF_SNAPSHOT`

  This function can support not following symlinks.

  Raises an auditing event `os.chflags` with arguments `path`, `flags`.

  Availability: Unix.

  New in version 3.3: The follow_symlinks argument.

  Changed in version 3.6: Accepts a path-like object.

`os.``chmod`(path, mode, *, dir_fd=None, follow_symlinks=True)

  Change the mode of path to the numeric mode. mode may take one of the following values (as defined in the `stat` module) or bitwise ORed combinations of them:

  - `stat.S_ISUID`

  - `stat.S_ISGID`

  - `stat.S_ENFMT`

  - `stat.S_ISVTX`

  - `stat.S_IREAD`

  - `stat.S_IWRITE`

  - `stat.S_IEXEC`

  - `stat.S_IRWXU`

  - `stat.S_IRUSR`

  - `stat.S_IWUSR`

  - `stat.S_IXUSR`

  - `stat.S_IRWXG`

  - `stat.S_IRGRP`

  - `stat.S_IWGRP`

  - `stat.S_IXGRP`

  - `stat.S_IRWXO`

  - `stat.S_IROTH`

  - `stat.S_IWOTH`

  - `stat.S_IXOTH`

  This function can support specifying a file descriptor, paths relative to directory descriptors and not following symlinks.

  Note

  Although Windows supports `chmod()`, you can only set the file’s read-only flag with it (via the `stat.S_IWRITE` and `stat.S_IREAD` constants or a corresponding integer value). All other bits are ignored.

  Raises an auditing event `os.chmod` with arguments `path`, `mode`, `dir_fd`.

  New in version 3.3: Added support for specifying path as an open file descriptor, and the dir_fd and follow_symlinks arguments.

  Changed in version 3.6: Accepts a path-like object.

`os.``chown`(path, uid, gid, *, dir_fd=None, follow_symlinks=True)

  Change the owner and group id of path to the numeric uid and gid. To leave one of the ids unchanged, set it to -1.

  This function can support specifying a file descriptor, paths relative to directory descriptors and not following symlinks.

  See `shutil.chown()` for a higher-level function that accepts names in addition to numeric ids.

  Raises an auditing event `os.chown` with arguments `path`, `uid`, `gid`, `dir_fd`.

  Availability: Unix.

  New in version 3.3: Added support for specifying path as an open file descriptor, and the dir_fd and follow_symlinks arguments.

  Changed in version 3.6: Supports a path-like object.

`os.``chroot`(path)

  Change the root directory of the current process to path.

  Availability: Unix.

  Changed in version 3.6: Accepts a path-like object.

`os.``fchdir`(fd)

  Change the current working directory to the directory represented by the file descriptor fd. The descriptor must refer to an opened directory, not an open file. As of Python 3.3, this is equivalent to `os.chdir(fd)`.

  Raises an auditing event `os.chdir` with argument `path`.

  Availability: Unix.

`os.``getcwd`()

  Return a string representing the current working directory.

`os.``getcwdb`()

  Return a bytestring representing the current working directory.

  Changed in version 3.8: The function now uses the UTF-8 encoding on Windows, rather than the ANSI code page: see PEP 529 for the rationale. The function is no longer deprecated on Windows.

`os.``lchflags`(path, flags)

  Set the flags of path to the numeric flags, like `chflags()`, but do not follow symbolic links. As of Python 3.3, this is equivalent to `os.chflags(path, flags, follow_symlinks=False)`.

  Raises an auditing event `os.chflags` with arguments `path`, `flags`.

  Availability: Unix.

  Changed in version 3.6: Accepts a path-like object.

`os.``lchmod`(path, mode)

  Change the mode of path to the numeric mode. If path is a symlink, this affects the symlink rather than the target. See the docs for `chmod()` for possible values of mode. As of Python 3.3, this is equivalent to `os.chmod(path, mode, follow_symlinks=False)`.

  Raises an auditing event `os.chmod` with arguments `path`, `mode`, `dir_fd`.

  Availability: Unix.

  Changed in version 3.6: Accepts a path-like object.

`os.``lchown`(path, uid, gid)

  Change the owner and group id of path to the numeric uid and gid. This function will not follow symbolic links. As of Python 3.3, this is equivalent to `os.chown(path, uid, gid, follow_symlinks=False)`.

  Raises an auditing event `os.chown` with arguments `path`, `uid`, `gid`, `dir_fd`.

  Availability: Unix.

  Changed in version 3.6: Accepts a path-like object.

`os.``link`(src, dst, *, src_dir_fd=None, dst_dir_fd=None, follow_symlinks=True)

  Create a hard link pointing to src named dst.

  This function can support specifying src_dir_fd and/or dst_dir_fd to supply paths relative to directory descriptors, and not following symlinks.

  Raises an auditing event `os.link` with arguments `src`, `dst`, `src_dir_fd`, `dst_dir_fd`.

  Availability: Unix, Windows.

  Changed in version 3.2: Added Windows support.

  New in version 3.3: Added the src_dir_fd, dst_dir_fd, and follow_symlinks arguments.

  Changed in version 3.6: Accepts a path-like object for src and dst.

`os.``listdir`(path='.')

  Return a list containing the names of the entries in the directory given by path. The list is in arbitrary order, and does not include the special entries `'.'` and `'..'` even if they are present in the directory. If a file is removed from or added to the directory during the call of this function, whether a name for that file be included is unspecified.

  path may be a path-like object. If path is of type `bytes` (directly or indirectly through the `PathLike` interface), the filenames returned will also be of type `bytes`; in all other circumstances, they will be of type `str`.

  This function can also support specifying a file descriptor; the file descriptor must refer to a directory.

  Raises an auditing event `os.listdir` with argument `path`.

  Note

  To encode `str` filenames to `bytes`, use `fsencode()`.

  See also

  The `scandir()` function returns directory entries along with file attribute information, giving better performance for many common use cases.

  Changed in version 3.2: The path parameter became optional.

  New in version 3.3: Added support for specifying path as an open file descriptor.

  Changed in version 3.6: Accepts a path-like object.

`os.``lstat`(path, *, dir_fd=None)

  Perform the equivalent of an `lstat()` system call on the given path. Similar to `stat()`, but does not follow symbolic links. Return a `stat_result` object.

  On platforms that do not support symbolic links, this is an alias for `stat()`.

  As of Python 3.3, this is equivalent to `os.stat(path, dir_fd=dir_fd, follow_symlinks=False)`.

  This function can also support paths relative to directory descriptors.

  See also

  The `stat()` function.

  Changed in version 3.2: Added support for Windows 6.0 (Vista) symbolic links.

  Changed in version 3.3: Added the dir_fd parameter.

  Changed in version 3.6: Accepts a path-like object.

  Changed in version 3.8: On Windows, now opens reparse points that represent another path (name surrogates), including symbolic links and directory junctions. Other kinds of reparse points are resolved by the operating system as for `stat()`.

`os.``mkdir`(path, mode=0o777, *, dir_fd=None)

  Create a directory named path with numeric mode mode.

  If the directory already exists, `FileExistsError` is raised. If a parent directory in the path does not exist, `FileNotFoundError` is raised.

  On some systems, mode is ignored. Where it is used, the current umask value is first masked out. If bits other than the last 9 (i.e. the last 3 digits of the octal representation of the mode) are set, their meaning is platform-dependent. On some platforms, they are ignored and you should call `chmod()` explicitly to set them.

  This function can also support paths relative to directory descriptors.

  It is also possible to create temporary directories; see the `tempfile` module’s `tempfile.mkdtemp()` function.

  Raises an auditing event `os.mkdir` with arguments `path`, `mode`, `dir_fd`.

  New in version 3.3: The dir_fd argument.

  Changed in version 3.6: Accepts a path-like object.

`os.``makedirs`(name, mode=0o777, exist_ok=False)

  Recursive directory creation function. Like `mkdir()`, but makes all intermediate-level directories needed to contain the leaf directory.

  The mode parameter is passed to `mkdir()` for creating the leaf directory; see the mkdir() description for how it is interpreted. To set the file permission bits of any newly-created parent directories you can set the umask before invoking `makedirs()`. The file permission bits of existing parent directories are not changed.

  If exist_ok is `False` (the default), an `FileExistsError` is raised if the target directory already exists.

  Note

  `makedirs()` will become confused if the path elements to create include `pardir` (eg. “..” on UNIX systems).

  This function handles UNC paths correctly.

  Raises an auditing event `os.mkdir` with arguments `path`, `mode`, `dir_fd`.

  New in version 3.2: The exist_ok parameter.

  Changed in version 3.4.1: Before Python 3.4.1, if exist_ok was `True` and the directory existed, `makedirs()` would still raise an error if mode did not match the mode of the existing directory. Since this behavior was impossible to implement safely, it was removed in Python 3.4.1. See bpo-21082.

  Changed in version 3.6: Accepts a path-like object.

  Changed in version 3.7: The mode argument no longer affects the file permission bits of newly-created intermediate-level directories.

`os.``mkfifo`(path, mode=0o666, *, dir_fd=None)

  Create a FIFO (a named pipe) named path with numeric mode mode. The current umask value is first masked out from the mode.

  This function can also support paths relative to directory descriptors.

  FIFOs are pipes that can be accessed like regular files. FIFOs exist until they are deleted (for example with `os.unlink()`). Generally, FIFOs are used as rendezvous between “client” and “server” type processes: the server opens the FIFO for reading, and the client opens it for writing. Note that `mkfifo()` doesn’t open the FIFO — it just creates the rendezvous point.

  Availability: Unix.

  New in version 3.3: The dir_fd argument.

  Changed in version 3.6: Accepts a path-like object.

`os.``mknod`(path, mode=0o600, device=0, *, dir_fd=None)

  Create a filesystem node (file, device special file or named pipe) named path. mode specifies both the permissions to use and the type of node to be created, being combined (bitwise OR) with one of `stat.S_IFREG`, `stat.S_IFCHR`, `stat.S_IFBLK`, and `stat.S_IFIFO` (those constants are available in `stat`). For `stat.S_IFCHR` and `stat.S_IFBLK`, device defines the newly created device special file (probably using `os.makedev()`), otherwise it is ignored.

  This function can also support paths relative to directory descriptors.

  Availability: Unix.

  New in version 3.3: The dir_fd argument.

  Changed in version 3.6: Accepts a path-like object.

`os.``major`(device)

  Extract the device major number from a raw device number (usually the `st_dev` or `st_rdev` field from `stat`).

`os.``minor`(device)

  Extract the device minor number from a raw device number (usually the `st_dev` or `st_rdev` field from `stat`).

`os.``makedev`(major, minor)

  Compose a raw device number from the major and minor device numbers.

`os.``pathconf`(path, name)

  Return system configuration information relevant to a named file. name specifies the configuration value to retrieve; it may be a string which is the name of a defined system value; these names are specified in a number of standards (POSIX.1, Unix 95, Unix 98, and others). Some platforms define additional names as well. The names known to the host operating system are given in the `pathconf_names` dictionary. For configuration variables not included in that mapping, passing an integer for name is also accepted.

  If name is a string and is not known, `ValueError` is raised. If a specific value for name is not supported by the host system, even if it is included in `pathconf_names`, an `OSError` is raised with `errno.EINVAL` for the error number.

  This function can support specifying a file descriptor.

  Availability: Unix.

  Changed in version 3.6: Accepts a path-like object.

`os.``pathconf_names`

  Dictionary mapping names accepted by `pathconf()` and `fpathconf()` to the integer values defined for those names by the host operating system. This can be used to determine the set of names known to the system.

  Availability: Unix.

`os.``readlink`(path, *, dir_fd=None)

  Return a string representing the path to which the symbolic link points. The result may be either an absolute or relative pathname; if it is relative, it may be converted to an absolute pathname using `os.path.join(os.path.dirname(path), result)`.

  If the path is a string object (directly or indirectly through a `PathLike` interface), the result will also be a string object, and the call may raise a UnicodeDecodeError. If the path is a bytes object (direct or indirectly), the result will be a bytes object.

  This function can also support paths relative to directory descriptors.

  When trying to resolve a path that may contain links, use `realpath()` to properly handle recursion and platform differences.

  Availability: Unix, Windows.

  Changed in version 3.2: Added support for Windows 6.0 (Vista) symbolic links.

  New in version 3.3: The dir_fd argument.

  Changed in version 3.6: Accepts a path-like object on Unix.

  Changed in version 3.8: Accepts a path-like object and a bytes object on Windows.

  Changed in version 3.8: Added support for directory junctions, and changed to return the substitution path (which typically includes `\\?\` prefix) rather than the optional “print name” field that was previously returned.

`os.``remove`(path, *, dir_fd=None)

  Remove (delete) the file path. If path is a directory, an `IsADirectoryError` is raised. Use `rmdir()` to remove directories. If the file does not exist, a `FileNotFoundError` is raised.

  This function can support paths relative to directory descriptors.

  On Windows, attempting to remove a file that is in use causes an exception to be raised; on Unix, the directory entry is removed but the storage allocated to the file is not made available until the original file is no longer in use.

  This function is semantically identical to `unlink()`.

  Raises an auditing event `os.remove` with arguments `path`, `dir_fd`.

  New in version 3.3: The dir_fd argument.

  Changed in version 3.6: Accepts a path-like object.

`os.``removedirs`(name)

  Remove directories recursively. Works like `rmdir()` except that, if the leaf directory is successfully removed, `removedirs()` tries to successively remove every parent directory mentioned in path until an error is raised (which is ignored, because it generally means that a parent directory is not empty). For example, `os.removedirs('foo/bar/baz')` will first remove the directory `'foo/bar/baz'`, and then remove `'foo/bar'` and `'foo'` if they are empty. Raises `OSError` if the leaf directory could not be successfully removed.

  Raises an auditing event `os.remove` with arguments `path`, `dir_fd`.

  Changed in version 3.6: Accepts a path-like object.

`os.``rename`(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

  Rename the file or directory src to dst. If dst exists, the operation will fail with an `OSError` subclass in a number of cases:

  On Windows, if dst exists a `FileExistsError` is always raised.

  On Unix, if src is a file and dst is a directory or vice-versa, an `IsADirectoryError` or a `NotADirectoryError` will be raised respectively. If both are directories and dst is empty, dst will be silently replaced. If dst is a non-empty directory, an `OSError` is raised. If both are files, dst it will be replaced silently if the user has permission. The operation may fail on some Unix flavors if src and dst are on different filesystems. If successful, the renaming will be an atomic operation (this is a POSIX requirement).

  This function can support specifying src_dir_fd and/or dst_dir_fd to supply paths relative to directory descriptors.

  If you want cross-platform overwriting of the destination, use `replace()`.

  Raises an auditing event `os.rename` with arguments `src`, `dst`, `src_dir_fd`, `dst_dir_fd`.

  New in version 3.3: The src_dir_fd and dst_dir_fd arguments.

  Changed in version 3.6: Accepts a path-like object for src and dst.

`os.``renames`(old, new)

  Recursive directory or file renaming function. Works like `rename()`, except creation of any intermediate directories needed to make the new pathname good is attempted first. After the rename, directories corresponding to rightmost path segments of the old name will be pruned away using `removedirs()`.

  Note

  This function can fail with the new directory structure made if you lack permissions needed to remove the leaf directory or file.

  Raises an auditing event `os.rename` with arguments `src`, `dst`, `src_dir_fd`, `dst_dir_fd`.

  Changed in version 3.6: Accepts a path-like object for old and new.

`os.``replace`(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

  Rename the file or directory src to dst. If dst is a non-empty directory, `OSError` will be raised. If dst exists and is a file, it will be replaced silently if the user has permission. The operation may fail if src and dst are on different filesystems. If successful, the renaming will be an atomic operation (this is a POSIX requirement).

  This function can support specifying src_dir_fd and/or dst_dir_fd to supply paths relative to directory descriptors.

  Raises an auditing event `os.rename` with arguments `src`, `dst`, `src_dir_fd`, `dst_dir_fd`.

  New in version 3.3.

  Changed in version 3.6: Accepts a path-like object for src and dst.

`os.``rmdir`(path, *, dir_fd=None)

  Remove (delete) the directory path. If the directory does not exist or is not empty, an `FileNotFoundError` or an `OSError` is raised respectively. In order to remove whole directory trees, `shutil.rmtree()` can be used.

  This function can support paths relative to directory descriptors.

  Raises an auditing event `os.rmdir` with arguments `path`, `dir_fd`.

  New in version 3.3: The dir_fd parameter.

  Changed in version 3.6: Accepts a path-like object.

`os.``scandir`(path='.')

  Return an iterator of `os.DirEntry` objects corresponding to the entries in the directory given by path. The entries are yielded in arbitrary order, and the special entries `'.'` and `'..'` are not included. If a file is removed from or added to the directory after creating the iterator, whether an entry for that file be included is unspecified.

  Using `scandir()` instead of `listdir()` can significantly increase the performance of code that also needs file type or file attribute information, because `os.DirEntry` objects expose this information if the operating system provides it when scanning a directory. All `os.DirEntry` methods may perform a system call, but `is_dir()` and `is_file()` usually only require a system call for symbolic links; `os.DirEntry.stat()` always requires a system call on Unix but only requires one for symbolic links on Windows.

  path may be a path-like object. If path is of type `bytes` (directly or indirectly through the `PathLike` interface), the type of the `name` and `path` attributes of each `os.DirEntry` will be `bytes`; in all other circumstances, they will be of type `str`.

  This function can also support specifying a file descriptor; the file descriptor must refer to a directory.

  Raises an auditing event `os.scandir` with argument `path`.

  The `scandir()` iterator supports the context manager protocol and has the following method:

  `scandir.``close`()

    Close the iterator and free acquired resources.

    This is called automatically when the iterator is exhausted or garbage collected, or when an error happens during iterating. However it is advisable to call it explicitly or use the `with` statement.

    New in version 3.6.

  The following example shows a simple use of `scandir()` to display all the files (excluding directories) in the given path that don’t start with `'.'`. The `entry.is_file()` call will generally not make an additional system call:

      with os.scandir(path) as it:
          for entry in it:
              if not entry.name.startswith('.') and entry.is_file():
                  print(entry.name)

  Note

  On Unix-based systems, `scandir()` uses the system’s opendir() and readdir() functions. On Windows, it uses the Win32 FindFirstFileW and FindNextFileW functions.

  New in version 3.5.

  New in version 3.6: Added support for the context manager protocol and the `close()` method. If a `scandir()` iterator is neither exhausted nor explicitly closed a `ResourceWarning` will be emitted in its destructor.

  The function accepts a path-like object.

  Changed in version 3.7: Added support for file descriptors on Unix.

class `os.``DirEntry`

  Object yielded by `scandir()` to expose the file path and other file attributes of a directory entry.

  `scandir()` will provide as much of this information as possible without making additional system calls. When a `stat()` or `lstat()` system call is made, the `os.DirEntry` object will cache the result.

  `os.DirEntry` instances are not intended to be stored in long-lived data structures; if you know the file metadata has changed or if a long time has elapsed since calling `scandir()`, call `os.stat(entry.path)` to fetch up-to-date information.

  Because the `os.DirEntry` methods can make operating system calls, they may also raise `OSError`. If you need very fine-grained control over errors, you can catch `OSError` when calling one of the `os.DirEntry` methods and handle as appropriate.

  To be directly usable as a path-like object, `os.DirEntry` implements the `PathLike` interface.

  Attributes and methods on a `os.DirEntry` instance are as follows:

  `name`

    The entry’s base filename, relative to the `scandir()` path argument.

    The `name` attribute will be `bytes` if the `scandir()` path argument is of type `bytes` and `str` otherwise. Use `fsdecode()` to decode byte filenames.

  `path`

    The entry’s full path name: equivalent to `os.path.join(scandir_path, entry.name)` where scandir_path is the `scandir()` path argument. The path is only absolute if the `scandir()` path argument was absolute. If the `scandir()` path argument was a file descriptor, the `path` attribute is the same as the `name` attribute.

    The `path` attribute will be `bytes` if the `scandir()` path argument is of type `bytes` and `str` otherwise. Use `fsdecode()` to decode byte filenames.

  `inode`()

    Return the inode number of the entry.

    The result is cached on the `os.DirEntry` object. Use `os.stat(entry.path, follow_symlinks=False).st_ino` to fetch up-to-date information.

    On the first, uncached call, a system call is required on Windows but not on Unix.

  `is_dir`(*, follow_symlinks=True)

    Return `True` if this entry is a directory or a symbolic link pointing to a directory; return `False` if the entry is or points to any other kind of file, or if it doesn’t exist anymore.

    If follow_symlinks is `False`, return `True` only if this entry is a directory (without following symlinks); return `False` if the entry is any other kind of file or if it doesn’t exist anymore.

    The result is cached on the `os.DirEntry` object, with a separate cache for follow_symlinks `True` and `False`. Call `os.stat()` along with `stat.S_ISDIR()` to fetch up-to-date information.

    On the first, uncached call, no system call is required in most cases. Specifically, for non-symlinks, neither Windows or Unix require a system call, except on certain Unix file systems, such as network file systems, that return `dirent.d_type == DT_UNKNOWN`. If the entry is a symlink, a system call will be required to follow the symlink unless follow_symlinks is `False`.

    This method can raise `OSError`, such as `PermissionError`, but `FileNotFoundError` is caught and not raised.

  `is_file`(*, follow_symlinks=True)

    Return `True` if this entry is a file or a symbolic link pointing to a file; return `False` if the entry is or points to a directory or other non-file entry, or if it doesn’t exist anymore.

    If follow_symlinks is `False`, return `True` only if this entry is a file (without following symlinks); return `False` if the entry is a directory or other non-file entry, or if it doesn’t exist anymore.

    The result is cached on the `os.DirEntry` object. Caching, system calls made, and exceptions raised are as per `is_dir()`.

  `is_symlink`()

    Return `True` if this entry is a symbolic link (even if broken); return `False` if the entry points to a directory or any kind of file, or if it doesn’t exist anymore.

    The result is cached on the `os.DirEntry` object. Call `os.path.islink()` to fetch up-to-date information.

    On the first, uncached call, no system call is required in most cases. Specifically, neither Windows or Unix require a system call, except on certain Unix file systems, such as network file systems, that return `dirent.d_type == DT_UNKNOWN`.

    This method can raise `OSError`, such as `PermissionError`, but `FileNotFoundError` is caught and not raised.

  `stat`(*, follow_symlinks=True)

    Return a `stat_result` object for this entry. This method follows symbolic links by default; to stat a symbolic link add the `follow_symlinks=False` argument.

    On Unix, this method always requires a system call. On Windows, it only requires a system call if follow_symlinks is `True` and the entry is a reparse point (for example, a symbolic link or directory junction).

    On Windows, the `st_ino`, `st_dev` and `st_nlink` attributes of the `stat_result` are always set to zero. Call `os.stat()` to get these attributes.

    The result is cached on the `os.DirEntry` object, with a separate cache for follow_symlinks `True` and `False`. Call `os.stat()` to fetch up-to-date information.

  Note that there is a nice correspondence between several attributes and methods of `os.DirEntry` and of `pathlib.Path`. In particular, the `name` attribute has the same meaning, as do the `is_dir()`, `is_file()`, `is_symlink()` and `stat()` methods.

  New in version 3.5.

  Changed in version 3.6: Added support for the `PathLike` interface. Added support for `bytes` paths on Windows.

`os.``stat`(path, *, dir_fd=None, follow_symlinks=True)

  Get the status of a file or a file descriptor. Perform the equivalent of a `stat()` system call on the given path. path may be specified as either a string or bytes – directly or indirectly through the `PathLike` interface – or as an open file descriptor. Return a `stat_result` object.

  This function normally follows symlinks; to stat a symlink add the argument `follow_symlinks=False`, or use `lstat()`.

  This function can support specifying a file descriptor and not following symlinks.

  On Windows, passing `follow_symlinks=False` will disable following all name-surrogate reparse points, which includes symlinks and directory junctions. Other types of reparse points that do not resemble links or that the operating system is unable to follow will be opened directly. When following a chain of multiple links, this may result in the original link being returned instead of the non-link that prevented full traversal. To obtain stat results for the final path in this case, use the `os.path.realpath()` function to resolve the path name as far as possible and call `lstat()` on the result. This does not apply to dangling symlinks or junction points, which will raise the usual exceptions.

  Example:

      >>> import os
      >>> statinfo = os.stat('somefile.txt')
      >>> statinfo
      os.stat_result(st_mode=33188, st_ino=7876932, st_dev=234881026,
      st_nlink=1, st_uid=501, st_gid=501, st_size=264, st_atime=1297230295,
      st_mtime=1297230027, st_ctime=1297230027)
      >>> statinfo.st_size
      264

  See also

  `fstat()` and `lstat()` functions.

  New in version 3.3: Added the dir_fd and follow_symlinks arguments, specifying a file descriptor instead of a path.

  Changed in version 3.6: Accepts a path-like object.

  Changed in version 3.8: On Windows, all reparse points that can be resolved by the operating system are now followed, and passing `follow_symlinks=False` disables following all name surrogate reparse points. If the operating system reaches a reparse point that it is not able to follow, stat now returns the information for the original path as if `follow_symlinks=False` had been specified instead of raising an error.

class `os.``stat_result`

  Object whose attributes correspond roughly to the members of the `stat` structure. It is used for the result of `os.stat()`, `os.fstat()` and `os.lstat()`.

  Attributes:

  `st_mode`

    File mode: file type and file mode bits (permissions).

  `st_ino`

    Platform dependent, but if non-zero, uniquely identifies the file for a given value of `st_dev`. Typically:

    - the inode number on Unix,

    - the file index on Windows

  `st_dev`

    Identifier of the device on which this file resides.

  `st_nlink`

    Number of hard links.

  `st_uid`

    User identifier of the file owner.

  `st_gid`

    Group identifier of the file owner.

  `st_size`

    Size of the file in bytes, if it is a regular file or a symbolic link. The size of a symbolic link is the length of the pathname it contains, without a terminating null byte.

  Timestamps:

  `st_atime`

    Time of most recent access expressed in seconds.

  `st_mtime`

    Time of most recent content modification expressed in seconds.

  `st_ctime`

    Platform dependent:

    - the time of most recent metadata change on Unix,

    - the time of creation on Windows, expressed in seconds.

  `st_atime_ns`

    Time of most recent access expressed in nanoseconds as an integer.

  `st_mtime_ns`

    Time of most recent content modification expressed in nanoseconds as an integer.

  `st_ctime_ns`

    Platform dependent:

    - the time of most recent metadata change on Unix,

    - the time of creation on Windows, expressed in nanoseconds as an integer.

  Note

  The exact meaning and resolution of the `st_atime`, `st_mtime`, and `st_ctime` attributes depend on the operating system and the file system. For example, on Windows systems using the FAT or FAT32 file systems, `st_mtime` has 2-second resolution, and `st_atime` has only 1-day resolution. See your operating system documentation for details.

  Similarly, although `st_atime_ns`, `st_mtime_ns`, and `st_ctime_ns` are always expressed in nanoseconds, many systems do not provide nanosecond precision. On systems that do provide nanosecond precision, the floating-point object used to store `st_atime`, `st_mtime`, and `st_ctime` cannot preserve all of it, and as such will be slightly inexact. If you need the exact timestamps you should always use `st_atime_ns`, `st_mtime_ns`, and `st_ctime_ns`.

  On some Unix systems (such as Linux), the following attributes may also be available:

  `st_blocks`

    Number of 512-byte blocks allocated for file. This may be smaller than `st_size`/512 when the file has holes.

  `st_blksize`

    “Preferred” blocksize for efficient file system I/O. Writing to a file in smaller chunks may cause an inefficient read-modify-rewrite.

  `st_rdev`

    Type of device if an inode device.

  `st_flags`

    User defined flags for file.

  On other Unix systems (such as FreeBSD), the following attributes may be available (but may be only filled out if root tries to use them):

  `st_gen`

    File generation number.

  `st_birthtime`

    Time of file creation.

  On Solaris and derivatives, the following attributes may also be available:

  `st_fstype`

    String that uniquely identifies the type of the filesystem that contains the file.

  On macOS systems, the following attributes may also be available:

  `st_rsize`

    Real size of the file.

  `st_creator`

    Creator of the file.

  `st_type`

    File type.

  On Windows systems, the following attributes are also available:

  `st_file_attributes`

    Windows file attributes: `dwFileAttributes` member of the `BY_HANDLE_FILE_INFORMATION` structure returned by `GetFileInformationByHandle()`. See the `FILE_ATTRIBUTE_*` constants in the `stat` module.

  `st_reparse_tag`

    When `st_file_attributes` has the `FILE_ATTRIBUTE_REPARSE_POINT` set, this field contains the tag identifying the type of reparse point. See the `IO_REPARSE_TAG_*` constants in the `stat` module.

  The standard module `stat` defines functions and constants that are useful for extracting information from a `stat` structure. (On Windows, some items are filled with dummy values.)

  For backward compatibility, a `stat_result` instance is also accessible as a tuple of at least 10 integers giving the most important (and portable) members of the `stat` structure, in the order `st_mode`, `st_ino`, `st_dev`, `st_nlink`, `st_uid`, `st_gid`, `st_size`, `st_atime`, `st_mtime`, `st_ctime`. More items may be added at the end by some implementations. For compatibility with older Python versions, accessing `stat_result` as a tuple always returns integers.

  New in version 3.3: Added the `st_atime_ns`, `st_mtime_ns`, and `st_ctime_ns` members.

  New in version 3.5: Added the `st_file_attributes` member on Windows.

  Changed in version 3.5: Windows now returns the file index as `st_ino` when available.

  New in version 3.7: Added the `st_fstype` member to Solaris/derivatives.

  New in version 3.8: Added the `st_reparse_tag` member on Windows.

  Changed in version 3.8: On Windows, the `st_mode` member now identifies special files as `S_IFCHR`, `S_IFIFO` or `S_IFBLK` as appropriate.

`os.``statvfs`(path)

  Perform a `statvfs()` system call on the given path. The return value is an object whose attributes describe the filesystem on the given path, and correspond to the members of the `statvfs` structure, namely: `f_bsize`, `f_frsize`, `f_blocks`, `f_bfree`, `f_bavail`, `f_files`, `f_ffree`, `f_favail`, `f_flag`, `f_namemax`, `f_fsid`.

  Two module-level constants are defined for the `f_flag` attribute’s bit-flags: if `ST_RDONLY` is set, the filesystem is mounted read-only, and if `ST_NOSUID` is set, the semantics of setuid/setgid bits are disabled or not supported.

  Additional module-level constants are defined for GNU/glibc based systems. These are `ST_NODEV` (disallow access to device special files), `ST_NOEXEC` (disallow program execution), `ST_SYNCHRONOUS` (writes are synced at once), `ST_MANDLOCK` (allow mandatory locks on an FS), `ST_WRITE` (write on file/directory/symlink), `ST_APPEND` (append-only file), `ST_IMMUTABLE` (immutable file), `ST_NOATIME` (do not update access times), `ST_NODIRATIME` (do not update directory access times), `ST_RELATIME` (update atime relative to mtime/ctime).

  This function can support specifying a file descriptor.

  Availability: Unix.

  Changed in version 3.2: The `ST_RDONLY` and `ST_NOSUID` constants were added.

  New in version 3.3: Added support for specifying path as an open file descriptor.

  Changed in version 3.4: The `ST_NODEV`, `ST_NOEXEC`, `ST_SYNCHRONOUS`, `ST_MANDLOCK`, `ST_WRITE`, `ST_APPEND`, `ST_IMMUTABLE`, `ST_NOATIME`, `ST_NODIRATIME`, and `ST_RELATIME` constants were added.

  Changed in version 3.6: Accepts a path-like object.

  New in version 3.7: Added `f_fsid`.

`os.``supports_dir_fd`

  A `set` object indicating which functions in the `os` module accept an open file descriptor for their dir_fd parameter. Different platforms provide different features, and the underlying functionality Python uses to implement the dir_fd parameter is not available on all platforms Python supports. For consistency’s sake, functions that may support dir_fd always allow specifying the parameter, but will throw an exception if the functionality is used when it’s not locally available. (Specifying `None` for dir_fd is always supported on all platforms.)

  To check whether a particular function accepts an open file descriptor for its dir_fd parameter, use the `in` operator on `supports_dir_fd`. As an example, this expression evaluates to `True` if `os.stat()` accepts open file descriptors for dir_fd on the local platform:

      os.stat in os.supports_dir_fd

  Currently dir_fd parameters only work on Unix platforms; none of them work on Windows.

  New in version 3.3.

`os.``supports_effective_ids`

  A `set` object indicating whether `os.access()` permits specifying `True` for its effective_ids parameter on the local platform. (Specifying `False` for effective_ids is always supported on all platforms.) If the local platform supports it, the collection will contain `os.access()`; otherwise it will be empty.

  This expression evaluates to `True` if `os.access()` supports `effective_ids=True` on the local platform:

      os.access in os.supports_effective_ids

  Currently effective_ids is only supported on Unix platforms; it does not work on Windows.

  New in version 3.3.

`os.``supports_fd`

  A `set` object indicating which functions in the `os` module permit specifying their path parameter as an open file descriptor on the local platform. Different platforms provide different features, and the underlying functionality Python uses to accept open file descriptors as path arguments is not available on all platforms Python supports.

  To determine whether a particular function permits specifying an open file descriptor for its path parameter, use the `in` operator on `supports_fd`. As an example, this expression evaluates to `True` if `os.chdir()` accepts open file descriptors for path on your local platform:

      os.chdir in os.supports_fd

  New in version 3.3.

`os.``supports_follow_symlinks`

  A `set` object indicating which functions in the `os` module accept `False` for their follow_symlinks parameter on the local platform. Different platforms provide different features, and the underlying functionality Python uses to implement follow_symlinks is not available on all platforms Python supports. For consistency’s sake, functions that may support follow_symlinks always allow specifying the parameter, but will throw an exception if the functionality is used when it’s not locally available. (Specifying `True` for follow_symlinks is always supported on all platforms.)

  To check whether a particular function accepts `False` for its follow_symlinks parameter, use the `in` operator on `supports_follow_symlinks`. As an example, this expression evaluates to `True` if you may specify `follow_symlinks=False` when calling `os.stat()` on the local platform:

      os.stat in os.supports_follow_symlinks

  New in version 3.3.

`os.``symlink`(src, dst, target_is_directory=False, *, dir_fd=None)

  Create a symbolic link pointing to src named dst.

  On Windows, a symlink represents either a file or a directory, and does not morph to the target dynamically. If the target is present, the type of the symlink will be created to match. Otherwise, the symlink will be created as a directory if target_is_directory is `True` or a file symlink (the default) otherwise. On non-Windows platforms, target_is_directory is ignored.

  This function can support paths relative to directory descriptors.

  Note

  On newer versions of Windows 10, unprivileged accounts can create symlinks if Developer Mode is enabled. When Developer Mode is not available/enabled, the SeCreateSymbolicLinkPrivilege privilege is required, or the process must be run as an administrator.

  `OSError` is raised when the function is called by an unprivileged user.

  Raises an auditing event `os.symlink` with arguments `src`, `dst`, `dir_fd`.

  Availability: Unix, Windows.

  Changed in version 3.2: Added support for Windows 6.0 (Vista) symbolic links.

  New in version 3.3: Added the dir_fd argument, and now allow target_is_directory on non-Windows platforms.

  Changed in version 3.6: Accepts a path-like object for src and dst.

  Changed in version 3.8: Added support for unelevated symlinks on Windows with Developer Mode.

`os.``sync`()

  Force write of everything to disk.

  Availability: Unix.

  New in version 3.3.

`os.``truncate`(path, length)

  Truncate the file corresponding to path, so that it is at most length bytes in size.

  This function can support specifying a file descriptor.

  Raises an auditing event `os.truncate` with arguments `path`, `length`.

  Availability: Unix, Windows.

  New in version 3.3.

  Changed in version 3.5: Added support for Windows

  Changed in version 3.6: Accepts a path-like object.

`os.``unlink`(path, *, dir_fd=None)

  Remove (delete) the file path. This function is semantically identical to `remove()`; the `unlink` name is its traditional Unix name. Please see the documentation for `remove()` for further information.

  Raises an auditing event `os.remove` with arguments `path`, `dir_fd`.

  New in version 3.3: The dir_fd parameter.

  Changed in version 3.6: Accepts a path-like object.

`os.``utime`(path, times=None, *, [ns, ]dir_fd=None, follow_symlinks=True)

  Set the access and modified times of the file specified by path.

  `utime()` takes two optional parameters, times and ns. These specify the times set on path and are used as follows:

  - If ns is specified, it must be a 2-tuple of the form `(atime_ns, mtime_ns)` where each member is an int expressing nanoseconds.

  - If times is not `None`, it must be a 2-tuple of the form `(atime, mtime)` where each member is an int or float expressing seconds.

  - If times is `None` and ns is unspecified, this is equivalent to specifying `ns=(atime_ns, mtime_ns)` where both times are the current time.

  It is an error to specify tuples for both times and ns.

  Note that the exact times you set here may not be returned by a subsequent `stat()` call, depending on the resolution with which your operating system records access and modification times; see `stat()`. The best way to preserve exact times is to use the st_atime_ns and st_mtime_ns fields from the `os.stat()` result object with the ns parameter to utime.

  This function can support specifying a file descriptor, paths relative to directory descriptors and not following symlinks.

  Raises an auditing event `os.utime` with arguments `path`, `times`, `ns`, `dir_fd`.

  New in version 3.3: Added support for specifying path as an open file descriptor, and the dir_fd, follow_symlinks, and ns parameters.

  Changed in version 3.6: Accepts a path-like object.

`os.``walk`(top, topdown=True, onerror=None, followlinks=False)

  Generate the file names in a directory tree by walking the tree either top-down or bottom-up. For each directory in the tree rooted at directory top (including top itself), it yields a 3-tuple `(dirpath, dirnames, filenames)`.

  dirpath is a string, the path to the directory. dirnames is a list of the names of the subdirectories in dirpath (excluding `'.'` and `'..'`). filenames is a list of the names of the non-directory files in dirpath. Note that the names in the lists contain no path components. To get a full path (which begins with top) to a file or directory in dirpath, do `os.path.join(dirpath, name)`. Whether or not the lists are sorted depends on the file system. If a file is removed from or added to the dirpath directory during generating the lists, whether a name for that file be included is unspecified.

  If optional argument topdown is `True` or not specified, the triple for a directory is generated before the triples for any of its subdirectories (directories are generated top-down). If topdown is `False`, the triple for a directory is generated after the triples for all of its subdirectories (directories are generated bottom-up). No matter the value of topdown, the list of subdirectories is retrieved before the tuples for the directory and its subdirectories are generated.

  When topdown is `True`, the caller can modify the dirnames list in-place (perhaps using `del` or slice assignment), and `walk()` will only recurse into the subdirectories whose names remain in dirnames; this can be used to prune the search, impose a specific order of visiting, or even to inform `walk()` about directories the caller creates or renames before it resumes `walk()` again. Modifying dirnames when topdown is `False` has no effect on the behavior of the walk, because in bottom-up mode the directories in dirnames are generated before dirpath itself is generated.

  By default, errors from the `scandir()` call are ignored. If optional argument onerror is specified, it should be a function; it will be called with one argument, an `OSError` instance. It can report the error to continue with the walk, or raise the exception to abort the walk. Note that the filename is available as the `filename` attribute of the exception object.

  By default, `walk()` will not walk down into symbolic links that resolve to directories. Set followlinks to `True` to visit directories pointed to by symlinks, on systems that support them.

  Note

  Be aware that setting followlinks to `True` can lead to infinite recursion if a link points to a parent directory of itself. `walk()` does not keep track of the directories it visited already.

  Note

  If you pass a relative pathname, don’t change the current working directory between resumptions of `walk()`. `walk()` never changes the current directory, and assumes that its caller doesn’t either.

  This example displays the number of bytes taken by non-directory files in each directory under the starting directory, except that it doesn’t look under any CVS subdirectory:

      import os
      from os.path import join, getsize
      for root, dirs, files in os.walk('python/Lib/email'):
          print(root, "consumes", end=" ")
          print(sum(getsize(join(root, name)) for name in files), end=" ")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

  In the next example (simple implementation of `shutil.rmtree()`), walking the tree bottom-up is essential, `rmdir()` doesn’t allow deleting a directory before the directory is empty:

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files in os.walk(top, topdown=False):
          for name in files:
              os.remove(os.path.join(root, name))
          for name in dirs:
              os.rmdir(os.path.join(root, name))

  Raises an auditing event `os.walk` with arguments `top`, `topdown`, `onerror`, `followlinks`.

  Changed in version 3.5: This function now calls `os.scandir()` instead of `os.listdir()`, making it faster by reducing the number of calls to `os.stat()`.

  Changed in version 3.6: Accepts a path-like object.

`os.``fwalk`(top='.', topdown=True, onerror=None, *, follow_symlinks=False, dir_fd=None)

  This behaves exactly like `walk()`, except that it yields a 4-tuple `(dirpath, dirnames, filenames, dirfd)`, and it supports `dir_fd`.

  dirpath, dirnames and filenames are identical to `walk()` output, and dirfd is a file descriptor referring to the directory dirpath.

  This function always supports paths relative to directory descriptors and not following symlinks. Note however that, unlike other functions, the `fwalk()` default value for follow_symlinks is `False`.

  Note

  Since `fwalk()` yields file descriptors, those are only valid until the next iteration step, so you should duplicate them (e.g. with `dup()`) if you want to keep them longer.

  This example displays the number of bytes taken by non-directory files in each directory under the starting directory, except that it doesn’t look under any CVS subdirectory:

      import os
      for root, dirs, files, rootfd in os.fwalk('python/Lib/email'):
          print(root, "consumes", end="")
          print(sum([os.stat(name, dir_fd=rootfd).st_size for name in files]),
                end="")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

  In the next example, walking the tree bottom-up is essential: `rmdir()` doesn’t allow deleting a directory before the directory is empty:

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files, rootfd in os.fwalk(top, topdown=False):
          for name in files:
              os.unlink(name, dir_fd=rootfd)
          for name in dirs:
              os.rmdir(name, dir_fd=rootfd)

  Raises an auditing event `os.fwalk` with arguments `top`, `topdown`, `onerror`, `follow_symlinks`, `dir_fd`.

  Availability: Unix.

  New in version 3.3.

  Changed in version 3.6: Accepts a path-like object.

  Changed in version 3.7: Added support for `bytes` paths.

`os.``memfd_create`(name[, flags=os.MFD_CLOEXEC])

  Create an anonymous file and return a file descriptor that refers to it. flags must be one of the `os.MFD_*` constants available on the system (or a bitwise ORed combination of them). By default, the new file descriptor is non-inheritable.

  The name supplied in name is used as a filename and will be displayed as the target of the corresponding symbolic link in the directory `/proc/self/fd/`. The displayed name is always prefixed with `memfd:` and serves only for debugging purposes. Names do not affect the behavior of the file descriptor, and as such multiple files can have the same name without any side effects.

  Availability: Linux 3.17 or newer with glibc 2.27 or newer.

  New in version 3.8.

`os.``MFD_CLOEXEC`
`os.``MFD_ALLOW_SEALING`
`os.``MFD_HUGETLB`
`os.``MFD_HUGE_SHIFT`
`os.``MFD_HUGE_MASK`
`os.``MFD_HUGE_64KB`
`os.``MFD_HUGE_512KB`
`os.``MFD_HUGE_1MB`
`os.``MFD_HUGE_2MB`
`os.``MFD_HUGE_8MB`
`os.``MFD_HUGE_16MB`
`os.``MFD_HUGE_32MB`
`os.``MFD_HUGE_256MB`
`os.``MFD_HUGE_512MB`
`os.``MFD_HUGE_1GB`
`os.``MFD_HUGE_2GB`
`os.``MFD_HUGE_16GB`

  These flags can be passed to `memfd_create()`.

  Availability: Linux 3.17 or newer with glibc 2.27 or newer. The `MFD_HUGE*` flags are only available since Linux 4.14.

  New in version 3.8.

### Linux extended attributes

New in version 3.3.

These functions are all available on Linux only.

`os.``getxattr`(path, attribute, *, follow_symlinks=True)

  Return the value of the extended filesystem attribute attribute for path. attribute can be bytes or str (directly or indirectly through the `PathLike` interface). If it is str, it is encoded with the filesystem encoding.

  This function can support specifying a file descriptor and not following symlinks.

  Raises an auditing event `os.getxattr` with arguments `path`, `attribute`.

  Changed in version 3.6: Accepts a path-like object for path and attribute.

`os.``listxattr`(path=None, *, follow_symlinks=True)

  Return a list of the extended filesystem attributes on path. The attributes in the list are represented as strings decoded with the filesystem encoding. If path is `None`, `listxattr()` will examine the current directory.

  This function can support specifying a file descriptor and not following symlinks.

  Raises an auditing event `os.listxattr` with argument `path`.

  Changed in version 3.6: Accepts a path-like object.

`os.``removexattr`(path, attribute, *, follow_symlinks=True)

  Removes the extended filesystem attribute attribute from path. attribute should be bytes or str (directly or indirectly through the `PathLike` interface). If it is a string, it is encoded with the filesystem encoding.

  This function can support specifying a file descriptor and not following symlinks.

  Raises an auditing event `os.removexattr` with arguments `path`, `attribute`.

  Changed in version 3.6: Accepts a path-like object for path and attribute.

`os.``setxattr`(path, attribute, value, flags=0, *, follow_symlinks=True)

  Set the extended filesystem attribute attribute on path to value. attribute must be a bytes or str with no embedded NULs (directly or indirectly through the `PathLike` interface). If it is a str, it is encoded with the filesystem encoding. flags may be `XATTR_REPLACE` or `XATTR_CREATE`. If `XATTR_REPLACE` is given and the attribute does not exist, `ENODATA` will be raised. If `XATTR_CREATE` is given and the attribute already exists, the attribute will not be created and `EEXISTS` will be raised.

  This function can support specifying a file descriptor and not following symlinks.

  Note

  A bug in Linux kernel versions less than 2.6.39 caused the flags argument to be ignored on some filesystems.

  Raises an auditing event `os.setxattr` with arguments `path`, `attribute`, `value`, `flags`.

  Changed in version 3.6: Accepts a path-like object for path and attribute.

`os.``XATTR_SIZE_MAX`

  The maximum size the value of an extended attribute can be. Currently, this is 64 KiB on Linux.

`os.``XATTR_CREATE`

  This is a possible value for the flags argument in `setxattr()`. It indicates the operation must create an attribute.

`os.``XATTR_REPLACE`

  This is a possible value for the flags argument in `setxattr()`. It indicates the operation must replace an existing attribute.

## Process Management

These functions may be used to create and manage processes.

The various `exec*` functions take a list of arguments for the new program loaded into the process. In each case, the first of these arguments is passed to the new program as its own name rather than as an argument a user may have typed on a command line. For the C programmer, this is the `argv[0]` passed to a program’s `main()`. For example, `os.execv('/bin/echo', ['foo', 'bar'])` will only print `bar` on standard output; `foo` will seem to be ignored.

`os.``abort`()

  Generate a `SIGABRT` signal to the current process. On Unix, the default behavior is to produce a core dump; on Windows, the process immediately returns an exit code of `3`. Be aware that calling this function will not call the Python signal handler registered for `SIGABRT` with `signal.signal()`.

`os.``add_dll_directory`(path)

  Add a path to the DLL search path.

  This search path is used when resolving dependencies for imported extension modules (the module itself is resolved through `sys.path`), and also by `ctypes`.

  Remove the directory by calling close() on the returned object or using it in a `with` statement.

  See the Microsoft documentation for more information about how DLLs are loaded.

  Raises an auditing event `os.add_dll_directory` with argument `path`.

  Availability: Windows.

  New in version 3.8: Previous versions of CPython would resolve DLLs using the default behavior for the current process. This led to inconsistencies, such as only sometimes searching `PATH` or the current working directory, and OS functions such as `AddDllDirectory` having no effect.

  In 3.8, the two primary ways DLLs are loaded now explicitly override the process-wide behavior to ensure consistency. See the porting notes for information on updating libraries.

`os.``execl`(path, arg0, arg1, ...)
`os.``execle`(path, arg0, arg1, ..., env)
`os.``execlp`(file, arg0, arg1, ...)
`os.``execlpe`(file, arg0, arg1, ..., env)
`os.``execv`(path, args)
`os.``execve`(path, args, env)
`os.``execvp`(file, args)
`os.``execvpe`(file, args, env)

  These functions all execute a new program, replacing the current process; they do not return. On Unix, the new executable is loaded into the current process, and will have the same process id as the caller. Errors will be reported as `OSError` exceptions.

  The current process is replaced immediately. Open file objects and descriptors are not flushed, so if there may be data buffered on these open files, you should flush them using `sys.stdout.flush()` or `os.fsync()` before calling an `exec*` function.

  The “l” and “v” variants of the `exec*` functions differ in how command-line arguments are passed. The “l” variants are perhaps the easiest to work with if the number of parameters is fixed when the code is written; the individual parameters simply become additional parameters to the `execl*()` functions. The “v” variants are good when the number of parameters is variable, with the arguments being passed in a list or tuple as the args parameter. In either case, the arguments to the child process should start with the name of the command being run, but this is not enforced.

  The variants which include a “p” near the end (`execlp()`, `execlpe()`, `execvp()`, and `execvpe()`) will use the `PATH` environment variable to locate the program file. When the environment is being replaced (using one of the `exec*e` variants, discussed in the next paragraph), the new environment is used as the source of the `PATH` variable. The other variants, `execl()`, `execle()`, `execv()`, and `execve()`, will not use the `PATH` variable to locate the executable; path must contain an appropriate absolute or relative path.

  For `execle()`, `execlpe()`, `execve()`, and `execvpe()` (note that these all end in “e”), the env parameter must be a mapping which is used to define the environment variables for the new process (these are used instead of the current process’ environment); the functions `execl()`, `execlp()`, `execv()`, and `execvp()` all cause the new process to inherit the environment of the current process.

  For `execve()` on some platforms, path may also be specified as an open file descriptor. This functionality may not be supported on your platform; you can check whether or not it is available using `os.supports_fd`. If it is unavailable, using it will raise a `NotImplementedError`.

  Raises an auditing event `os.exec` with arguments `path`, `args`, `env`.

  Availability: Unix, Windows.

  New in version 3.3: Added support for specifying path as an open file descriptor for `execve()`.

  Changed in version 3.6: Accepts a path-like object.

`os.``_exit`(n)

  Exit the process with status n, without calling cleanup handlers, flushing stdio buffers, etc.

  Note

  The standard way to exit is `sys.exit(n)`. `_exit()` should normally only be used in the child process after a `fork()`.

The following exit codes are defined and can be used with `_exit()`, although they are not required. These are typically used for system programs written in Python, such as a mail server’s external command delivery program.

Note

Some of these may not be available on all Unix platforms, since there is some variation. These constants are defined where they are defined by the underlying platform.

`os.``EX_OK`

  Exit code that means no error occurred.

  Availability: Unix.

`os.``EX_USAGE`

  Exit code that means the command was used incorrectly, such as when the wrong number of arguments are given.

  Availability: Unix.

`os.``EX_DATAERR`

  Exit code that means the input data was incorrect.

  Availability: Unix.

`os.``EX_NOINPUT`

  Exit code that means an input file did not exist or was not readable.

  Availability: Unix.

`os.``EX_NOUSER`

  Exit code that means a specified user did not exist.

  Availability: Unix.

`os.``EX_NOHOST`

  Exit code that means a specified host did not exist.

  Availability: Unix.

`os.``EX_UNAVAILABLE`

  Exit code that means that a required service is unavailable.

  Availability: Unix.

`os.``EX_SOFTWARE`

  Exit code that means an internal software error was detected.

  Availability: Unix.

`os.``EX_OSERR`

  Exit code that means an operating system error was detected, such as the inability to fork or create a pipe.

  Availability: Unix.

`os.``EX_OSFILE`

  Exit code that means some system file did not exist, could not be opened, or had some other kind of error.

  Availability: Unix.

`os.``EX_CANTCREAT`

  Exit code that means a user specified output file could not be created.

  Availability: Unix.

`os.``EX_IOERR`

  Exit code that means that an error occurred while doing I/O on some file.

  Availability: Unix.

`os.``EX_TEMPFAIL`

  Exit code that means a temporary failure occurred. This indicates something that may not really be an error, such as a network connection that couldn’t be made during a retryable operation.

  Availability: Unix.

`os.``EX_PROTOCOL`

  Exit code that means that a protocol exchange was illegal, invalid, or not understood.

  Availability: Unix.

`os.``EX_NOPERM`

  Exit code that means that there were insufficient permissions to perform the operation (but not intended for file system problems).

  Availability: Unix.

`os.``EX_CONFIG`

  Exit code that means that some kind of configuration error occurred.

  Availability: Unix.

`os.``EX_NOTFOUND`

  Exit code that means something like “an entry was not found”.

  Availability: Unix.

`os.``fork`()

  Fork a child process. Return `0` in the child and the child’s process id in the parent. If an error occurs `OSError` is raised.

  Note that some platforms including FreeBSD <= 6.3 and Cygwin have known issues when using `fork()` from a thread.

  Raises an auditing event `os.fork` with no arguments.

  Changed in version 3.8: Calling `fork()` in a subinterpreter is no longer supported (`RuntimeError` is raised).

  Warning

  See `ssl` for applications that use the SSL module with fork().

  Availability: Unix.

`os.``forkpty`()

  Fork a child process, using a new pseudo-terminal as the child’s controlling terminal. Return a pair of `(pid, fd)`, where pid is `0` in the child, the new child’s process id in the parent, and fd is the file descriptor of the master end of the pseudo-terminal. For a more portable approach, use the `pty` module. If an error occurs `OSError` is raised.

  Raises an auditing event `os.forkpty` with no arguments.

  Changed in version 3.8: Calling `forkpty()` in a subinterpreter is no longer supported (`RuntimeError` is raised).

  Availability: some flavors of Unix.

`os.``kill`(pid, sig)

  Send signal sig to the process pid. Constants for the specific signals available on the host platform are defined in the `signal` module.

  Windows: The `signal.CTRL_C_EVENT` and `signal.CTRL_BREAK_EVENT` signals are special signals which can only be sent to console processes which share a common console window, e.g., some subprocesses. Any other value for sig will cause the process to be unconditionally killed by the TerminateProcess API, and the exit code will be set to sig. The Windows version of `kill()` additionally takes process handles to be killed.

  See also `signal.pthread_kill()`.

  Raises an auditing event `os.kill` with arguments `pid`, `sig`.

  New in version 3.2: Windows support.

`os.``killpg`(pgid, sig)

  Send the signal sig to the process group pgid.

  Raises an auditing event `os.killpg` with arguments `pgid`, `sig`.

  Availability: Unix.

`os.``nice`(increment)

  Add increment to the process’s “niceness”. Return the new niceness.

  Availability: Unix.

`os.``pidfd_open`(pid, flags=0)

  Return a file descriptor referring to the process pid. This descriptor can be used to perform process management without races and signals. The flags argument is provided for future extensions; no flag values are currently defined.

  See the pidfd_open(2) man page for more details.

  Availability: Linux 5.3+

  New in version 3.9.

`os.``plock`(op)

  Lock program segments into memory. The value of op (defined in `<sys/lock.h>`) determines which segments are locked.

  Availability: Unix.

`os.``popen`(cmd, mode='r', buffering=-1)

  Open a pipe to or from command cmd. The return value is an open file object connected to the pipe, which can be read or written depending on whether mode is `'r'` (default) or `'w'`. The buffering argument has the same meaning as the corresponding argument to the built-in `open()` function. The returned file object reads or writes text strings rather than bytes.

  The `close` method returns `None` if the subprocess exited successfully, or the subprocess’s return code if there was an error. On POSIX systems, if the return code is positive it represents the return value of the process left-shifted by one byte. If the return code is negative, the process was terminated by the signal given by the negated value of the return code. (For example, the return value might be `- signal.SIGKILL` if the subprocess was killed.) On Windows systems, the return value contains the signed integer return code from the child process.

  On Unix, `waitstatus_to_exitcode()` can be used to convert the `close` method result (exit status) into an exit code if it is not `None`. On Windows, the `close` method result is directly the exit code (or `None`).

  This is implemented using `subprocess.Popen`; see that class’s documentation for more powerful ways to manage and communicate with subprocesses.

`os.``posix_spawn`(path, argv, env, *, file_actions=None, setpgroup=None, resetids=False, setsid=False, setsigmask=(), setsigdef=(), scheduler=None)

  Wraps the `posix_spawn()` C library API for use from Python.

  Most users should use `subprocess.run()` instead of `posix_spawn()`.

  The positional-only arguments path, args, and env are similar to `execve()`.

  The path parameter is the path to the executable file. The path should contain a directory. Use `posix_spawnp()` to pass an executable file without directory.

  The file_actions argument may be a sequence of tuples describing actions to take on specific file descriptors in the child process between the C library implementation’s `fork()` and `exec()` steps. The first item in each tuple must be one of the three type indicator listed below describing the remaining tuple elements:

  `os.``POSIX_SPAWN_OPEN`

    (`os.POSIX_SPAWN_OPEN`, fd, path, flags, mode)

    Performs `os.dup2(os.open(path, flags, mode), fd)`.

  `os.``POSIX_SPAWN_CLOSE`

    (`os.POSIX_SPAWN_CLOSE`, fd)

    Performs `os.close(fd)`.

  `os.``POSIX_SPAWN_DUP2`

    (`os.POSIX_SPAWN_DUP2`, fd, new_fd)

    Performs `os.dup2(fd, new_fd)`.

  These tuples correspond to the C library `posix_spawn_file_actions_addopen()`, `posix_spawn_file_actions_addclose()`, and `posix_spawn_file_actions_adddup2()` API calls used to prepare for the `posix_spawn()` call itself.

  The setpgroup argument will set the process group of the child to the value specified. If the value specified is 0, the child’s process group ID will be made the same as its process ID. If the value of setpgroup is not set, the child will inherit the parent’s process group ID. This argument corresponds to the C library `POSIX_SPAWN_SETPGROUP` flag.

  If the resetids argument is `True` it will reset the effective UID and GID of the child to the real UID and GID of the parent process. If the argument is `False`, then the child retains the effective UID and GID of the parent. In either case, if the set-user-ID and set-group-ID permission bits are enabled on the executable file, their effect will override the setting of the effective UID and GID. This argument corresponds to the C library `POSIX_SPAWN_RESETIDS` flag.

  If the setsid argument is `True`, it will create a new session ID for posix_spawn. setsid requires `POSIX_SPAWN_SETSID` or `POSIX_SPAWN_SETSID_NP` flag. Otherwise, `NotImplementedError` is raised.

  The setsigmask argument will set the signal mask to the signal set specified. If the parameter is not used, then the child inherits the parent’s signal mask. This argument corresponds to the C library `POSIX_SPAWN_SETSIGMASK` flag.

  The sigdef argument will reset the disposition of all signals in the set specified. This argument corresponds to the C library `POSIX_SPAWN_SETSIGDEF` flag.

  The scheduler argument must be a tuple containing the (optional) scheduler policy and an instance of `sched_param` with the scheduler parameters. A value of `None` in the place of the scheduler policy indicates that is not being provided. This argument is a combination of the C library `POSIX_SPAWN_SETSCHEDPARAM` and `POSIX_SPAWN_SETSCHEDULER` flags.

  Raises an auditing event `os.posix_spawn` with arguments `path`, `argv`, `env`.

  New in version 3.8.

  Availability: Unix.

`os.``posix_spawnp`(path, argv, env, *, file_actions=None, setpgroup=None, resetids=False, setsid=False, setsigmask=(), setsigdef=(), scheduler=None)

  Wraps the `posix_spawnp()` C library API for use from Python.

  Similar to `posix_spawn()` except that the system searches for the executable file in the list of directories specified by the `PATH` environment variable (in the same way as for `execvp(3)`).

  Raises an auditing event `os.posix_spawn` with arguments `path`, `argv`, `env`.

  New in version 3.8.

  Availability: See `posix_spawn()` documentation.

`os.``register_at_fork`(*, before=None, after_in_parent=None, after_in_child=None)

  Register callables to be executed when a new child process is forked using `os.fork()` or similar process cloning APIs. The parameters are optional and keyword-only. Each specifies a different call point.

  - before is a function called before forking a child process.

  - after_in_parent is a function called from the parent process after forking a child process.

  - after_in_child is a function called from the child process.

  These calls are only made if control is expected to return to the Python interpreter. A typical `subprocess` launch will not trigger them as the child is not going to re-enter the interpreter.

  Functions registered for execution before forking are called in reverse registration order. Functions registered for execution after forking (either in the parent or in the child) are called in registration order.

  Note that `fork()` calls made by third-party C code may not call those functions, unless it explicitly calls `PyOS_BeforeFork()`, `PyOS_AfterFork_Parent()` and `PyOS_AfterFork_Child()`.

  There is no way to unregister a function.

  Availability: Unix.

  New in version 3.7.

`os.``spawnl`(mode, path, ...)
`os.``spawnle`(mode, path, ..., env)
`os.``spawnlp`(mode, file, ...)
`os.``spawnlpe`(mode, file, ..., env)
`os.``spawnv`(mode, path, args)
`os.``spawnve`(mode, path, args, env)
`os.``spawnvp`(mode, file, args)
`os.``spawnvpe`(mode, file, args, env)

  Execute the program path in a new process.

  (Note that the `subprocess` module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using these functions. Check especially the Replacing Older Functions with the subprocess Module section.)

  If mode is `P_NOWAIT`, this function returns the process id of the new process; if mode is `P_WAIT`, returns the process’s exit code if it exits normally, or `-signal`, where signal is the signal that killed the process. On Windows, the process id will actually be the process handle, so can be used with the `waitpid()` function.

  Note on VxWorks, this function doesn’t return `-signal` when the new process is killed. Instead it raises OSError exception.

  The “l” and “v” variants of the `spawn*` functions differ in how command-line arguments are passed. The “l” variants are perhaps the easiest to work with if the number of parameters is fixed when the code is written; the individual parameters simply become additional parameters to the `spawnl*()` functions. The “v” variants are good when the number of parameters is variable, with the arguments being passed in a list or tuple as the args parameter. In either case, the arguments to the child process must start with the name of the command being run.

  The variants which include a second “p” near the end (`spawnlp()`, `spawnlpe()`, `spawnvp()`, and `spawnvpe()`) will use the `PATH` environment variable to locate the program file. When the environment is being replaced (using one of the `spawn*e` variants, discussed in the next paragraph), the new environment is used as the source of the `PATH` variable. The other variants, `spawnl()`, `spawnle()`, `spawnv()`, and `spawnve()`, will not use the `PATH` variable to locate the executable; path must contain an appropriate absolute or relative path.

  For `spawnle()`, `spawnlpe()`, `spawnve()`, and `spawnvpe()` (note that these all end in “e”), the env parameter must be a mapping which is used to define the environment variables for the new process (they are used instead of the current process’ environment); the functions `spawnl()`, `spawnlp()`, `spawnv()`, and `spawnvp()` all cause the new process to inherit the environment of the current process. Note that keys and values in the env dictionary must be strings; invalid keys or values will cause the function to fail, with a return value of `127`.

  As an example, the following calls to `spawnlp()` and `spawnvpe()` are equivalent:

      import os
      os.spawnlp(os.P_WAIT, 'cp', 'cp', 'index.html', '/dev/null')

      L = ['cp', 'index.html', '/dev/null']
      os.spawnvpe(os.P_WAIT, 'cp', L, os.environ)

  Raises an auditing event `os.spawn` with arguments `mode`, `path`, `args`, `env`.

  Availability: Unix, Windows. `spawnlp()`, `spawnlpe()`, `spawnvp()` and `spawnvpe()` are not available on Windows. `spawnle()` and `spawnve()` are not thread-safe on Windows; we advise you to use the `subprocess` module instead.

  Changed in version 3.6: Accepts a path-like object.

`os.``P_NOWAIT`
`os.``P_NOWAITO`

  Possible values for the mode parameter to the `spawn*` family of functions. If either of these values is given, the `spawn*()` functions will return as soon as the new process has been created, with the process id as the return value.

  Availability: Unix, Windows.

`os.``P_WAIT`

  Possible value for the mode parameter to the `spawn*` family of functions. If this is given as mode, the `spawn*()` functions will not return until the new process has run to completion and will return the exit code of the process the run is successful, or `-signal` if a signal kills the process.

  Availability: Unix, Windows.

`os.``P_DETACH`
`os.``P_OVERLAY`

  Possible values for the mode parameter to the `spawn*` family of functions. These are less portable than those listed above. `P_DETACH` is similar to `P_NOWAIT`, but the new process is detached from the console of the calling process. If `P_OVERLAY` is used, the current process will be replaced; the `spawn*` function will not return.

  Availability: Windows.

`os.``startfile`(path[, operation])

  Start a file with its associated application.

  When operation is not specified or `'open'`, this acts like double-clicking the file in Windows Explorer, or giving the file name as an argument to the start command from the interactive command shell: the file is opened with whatever application (if any) its extension is associated.

  When another operation is given, it must be a “command verb” that specifies what should be done with the file. Common verbs documented by Microsoft are `'print'` and `'edit'` (to be used on files) as well as `'explore'` and `'find'` (to be used on directories).

  `startfile()` returns as soon as the associated application is launched. There is no option to wait for the application to close, and no way to retrieve the application’s exit status. The path parameter is relative to the current directory. If you want to use an absolute path, make sure the first character is not a slash (`'/'`); the underlying Win32 `ShellExecute()` function doesn’t work if it is. Use the `os.path.normpath()` function to ensure that the path is properly encoded for Win32.

  To reduce interpreter startup overhead, the Win32 `ShellExecute()` function is not resolved until this function is first called. If the function cannot be resolved, `NotImplementedError` will be raised.

  Raises an auditing event `os.startfile` with arguments `path`, `operation`.

  Availability: Windows.

`os.``system`(command)

  Execute the command (a string) in a subshell. This is implemented by calling the Standard C function `system()`, and has the same limitations. Changes to `sys.stdin`, etc. are not reflected in the environment of the executed command. If command generates any output, it will be sent to the interpreter standard output stream. The C standard does not specify the meaning of the return value of the C function, so the return value of the Python function is system-dependent.

  On Unix, the return value is the exit status of the process encoded in the format specified for `wait()`.

  On Windows, the return value is that returned by the system shell after running command. The shell is given by the Windows environment variable `COMSPEC`: it is usually cmd.exe, which returns the exit status of the command run; on systems using a non-native shell, consult your shell documentation.

  The `subprocess` module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the `subprocess` documentation for some helpful recipes.

  On Unix, `waitstatus_to_exitcode()` can be used to convert the result (exit status) into an exit code. On Windows, the result is directly the exit code.

  Raises an auditing event `os.system` with argument `command`.

  Availability: Unix, Windows.

`os.``times`()

  Returns the current global process times. The return value is an object with five attributes:

  - `user` - user time

  - `system` - system time

  - `children_user` - user time of all child processes

  - `children_system` - system time of all child processes

  - `elapsed` - elapsed real time since a fixed point in the past

  For backwards compatibility, this object also behaves like a five-tuple containing `user`, `system`, `children_user`, `children_system`, and `elapsed` in that order.

  See the Unix manual page times(2) and times(3) manual page on Unix or the GetProcessTimes MSDN on Windows. On Windows, only `user` and `system` are known; the other attributes are zero.

  Availability: Unix, Windows.

  Changed in version 3.3: Return type changed from a tuple to a tuple-like object with named attributes.

`os.``wait`()

  Wait for completion of a child process, and return a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.

  `waitstatus_to_exitcode()` can be used to convert the exit status into an exit code.

  Availability: Unix.

  See also

  `waitpid()` can be used to wait for the completion of a specific child process and has more options.

`os.``waitid`(idtype, id, options)

  Wait for the completion of one or more child processes. idtype can be `P_PID`, `P_PGID`, `P_ALL`, or `P_PIDFD` on Linux. id specifies the pid to wait on. options is constructed from the ORing of one or more of `WEXITED`, `WSTOPPED` or `WCONTINUED` and additionally may be ORed with `WNOHANG` or `WNOWAIT`. The return value is an object representing the data contained in the `siginfo_t` structure, namely: `si_pid`, `si_uid`, `si_signo`, `si_status`, `si_code` or `None` if `WNOHANG` is specified and there are no children in a waitable state.

  Availability: Unix.

  New in version 3.3.

`os.``P_PID`
`os.``P_PGID`
`os.``P_ALL`

  These are the possible values for idtype in `waitid()`. They affect how id is interpreted.

  Availability: Unix.

  New in version 3.3.

`os.``P_PIDFD`

  This is a Linux-specific idtype that indicates that id is a file descriptor that refers to a process.

  Availability: Linux 5.4+

  New in version 3.9.

`os.``WEXITED`
`os.``WSTOPPED`
`os.``WNOWAIT`

  Flags that can be used in options in `waitid()` that specify what child signal to wait for.

  Availability: Unix.

  New in version 3.3.

`os.``CLD_EXITED`
`os.``CLD_KILLED`
`os.``CLD_DUMPED`
`os.``CLD_TRAPPED`
`os.``CLD_STOPPED`
`os.``CLD_CONTINUED`

  These are the possible values for `si_code` in the result returned by `waitid()`.

  Availability: Unix.

  New in version 3.3.

  Changed in version 3.9: Added `CLD_KILLED` and `CLD_STOPPED` values.

`os.``waitpid`(pid, options)

  The details of this function differ on Unix and Windows.

  On Unix: Wait for completion of a child process given by process id pid, and return a tuple containing its process id and exit status indication (encoded as for `wait()`). The semantics of the call are affected by the value of the integer options, which should be `0` for normal operation.

  If pid is greater than `0`, `waitpid()` requests status information for that specific process. If pid is `0`, the request is for the status of any child in the process group of the current process. If pid is `-1`, the request pertains to any child of the current process. If pid is less than `-1`, status is requested for any process in the process group `-pid` (the absolute value of pid).

  An `OSError` is raised with the value of errno when the syscall returns -1.

  On Windows: Wait for completion of a process given by process handle pid, and return a tuple containing pid, and its exit status shifted left by 8 bits (shifting makes cross-platform use of the function easier). A pid less than or equal to `0` has no special meaning on Windows, and raises an exception. The value of integer options has no effect. pid can refer to any process whose id is known, not necessarily a child process. The `spawn*` functions called with `P_NOWAIT` return suitable process handles.

  `waitstatus_to_exitcode()` can be used to convert the exit status into an exit code.

  Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the function now retries the system call instead of raising an `InterruptedError` exception (see PEP 475 for the rationale).

`os.``wait3`(options)

  Similar to `waitpid()`, except no process id argument is given and a 3-element tuple containing the child’s process id, exit status indication, and resource usage information is returned. Refer to `resource`.`getrusage()` for details on resource usage information. The option argument is the same as that provided to `waitpid()` and `wait4()`.

  `waitstatus_to_exitcode()` can be used to convert the exit status into an exitcode.

  Availability: Unix.

`os.``wait4`(pid, options)

  Similar to `waitpid()`, except a 3-element tuple, containing the child’s process id, exit status indication, and resource usage information is returned. Refer to `resource`.`getrusage()` for details on resource usage information. The arguments to `wait4()` are the same as those provided to `waitpid()`.

  `waitstatus_to_exitcode()` can be used to convert the exit status into an exitcode.

  Availability: Unix.

`os.``waitstatus_to_exitcode`(status)

  Convert a wait status to an exit code.

  On Unix:

  - If the process exited normally (if `WIFEXITED(status)` is true), return the process exit status (return `WEXITSTATUS(status)`): result greater than or equal to 0.

  - If the process was terminated by a signal (if `WIFSIGNALED(status)` is true), return `-signum` where signum is the number of the signal that caused the process to terminate (return `-WTERMSIG(status)`): result less than 0.

  - Otherwise, raise a `ValueError`.

  On Windows, return status shifted right by 8 bits.

  On Unix, if the process is being traced or if `waitpid()` was called with `WUNTRACED` option, the caller must first check if `WIFSTOPPED(status)` is true. This function must not be called if `WIFSTOPPED(status)` is true.

  See also

  `WIFEXITED()`, `WEXITSTATUS()`, `WIFSIGNALED()`, `WTERMSIG()`, `WIFSTOPPED()`, `WSTOPSIG()` functions.

  New in version 3.9.

`os.``WNOHANG`

  The option for `waitpid()` to return immediately if no child process status is available immediately. The function returns `(0, 0)` in this case.

  Availability: Unix.

`os.``WCONTINUED`

  This option causes child processes to be reported if they have been continued from a job control stop since their status was last reported.

  Availability: some Unix systems.

`os.``WUNTRACED`

  This option causes child processes to be reported if they have been stopped but their current state has not been reported since they were stopped.

  Availability: Unix.

The following functions take a process status code as returned by `system()`, `wait()`, or `waitpid()` as a parameter. They may be used to determine the disposition of a process.

`os.``WCOREDUMP`(status)

  Return `True` if a core dump was generated for the process, otherwise return `False`.

  This function should be employed only if `WIFSIGNALED()` is true.

  Availability: Unix.

`os.``WIFCONTINUED`(status)

  Return `True` if a stopped child has been resumed by delivery of `SIGCONT` (if the process has been continued from a job control stop), otherwise return `False`.

  See `WCONTINUED` option.

  Availability: Unix.

`os.``WIFSTOPPED`(status)

  Return `True` if the process was stopped by delivery of a signal, otherwise return `False`.

  `WIFSTOPPED()` only returns `True` if the `waitpid()` call was done using `WUNTRACED` option or when the process is being traced (see ptrace(2)).

  Availability: Unix.

`os.``WIFSIGNALED`(status)

  Return `True` if the process was terminated by a signal, otherwise return `False`.

  Availability: Unix.

`os.``WIFEXITED`(status)

  Return `True` if the process exited terminated normally, that is, by calling `exit()` or `_exit()`, or by returning from `main()`; otherwise return `False`.

  Availability: Unix.

`os.``WEXITSTATUS`(status)

  Return the process exit status.

  This function should be employed only if `WIFEXITED()` is true.

  Availability: Unix.

`os.``WSTOPSIG`(status)

  Return the signal which caused the process to stop.

  This function should be employed only if `WIFSTOPPED()` is true.

  Availability: Unix.

`os.``WTERMSIG`(status)

  Return the number of the signal that caused the process to terminate.

  This function should be employed only if `WIFSIGNALED()` is true.

  Availability: Unix.

## Interface to the scheduler

These functions control how a process is allocated CPU time by the operating system. They are only available on some Unix platforms. For more detailed information, consult your Unix manpages.

New in version 3.3.

The following scheduling policies are exposed if they are supported by the operating system.

`os.``SCHED_OTHER`

  The default scheduling policy.

`os.``SCHED_BATCH`

  Scheduling policy for CPU-intensive processes that tries to preserve interactivity on the rest of the computer.

`os.``SCHED_IDLE`

  Scheduling policy for extremely low priority background tasks.

`os.``SCHED_SPORADIC`

  Scheduling policy for sporadic server programs.

`os.``SCHED_FIFO`

  A First In First Out scheduling policy.

`os.``SCHED_RR`

  A round-robin scheduling policy.

`os.``SCHED_RESET_ON_FORK`

  This flag can be OR’ed with any other scheduling policy. When a process with this flag set forks, its child’s scheduling policy and priority are reset to the default.

class `os.``sched_param`(sched_priority)

  This class represents tunable scheduling parameters used in `sched_setparam()`, `sched_setscheduler()`, and `sched_getparam()`. It is immutable.

  At the moment, there is only one possible parameter:

  `sched_priority`

    The scheduling priority for a scheduling policy.

`os.``sched_get_priority_min`(policy)

  Get the minimum priority value for policy. policy is one of the scheduling policy constants above.

`os.``sched_get_priority_max`(policy)

  Get the maximum priority value for policy. policy is one of the scheduling policy constants above.

`os.``sched_setscheduler`(pid, policy, param)

  Set the scheduling policy for the process with PID pid. A pid of 0 means the calling process. policy is one of the scheduling policy constants above. param is a `sched_param` instance.

`os.``sched_getscheduler`(pid)

  Return the scheduling policy for the process with PID pid. A pid of 0 means the calling process. The result is one of the scheduling policy constants above.

`os.``sched_setparam`(pid, param)

  Set the scheduling parameters for the process with PID pid. A pid of 0 means the calling process. param is a `sched_param` instance.

`os.``sched_getparam`(pid)

  Return the scheduling parameters as a `sched_param` instance for the process with PID pid. A pid of 0 means the calling process.

`os.``sched_rr_get_interval`(pid)

  Return the round-robin quantum in seconds for the process with PID pid. A pid of 0 means the calling process.

`os.``sched_yield`()

  Voluntarily relinquish the CPU.

`os.``sched_setaffinity`(pid, mask)

  Restrict the process with PID pid (or the current process if zero) to a set of CPUs. mask is an iterable of integers representing the set of CPUs to which the process should be restricted.

`os.``sched_getaffinity`(pid)

  Return the set of CPUs the process with PID pid (or the current process if zero) is restricted to.

## Miscellaneous System Information

`os.``confstr`(name)

  Return string-valued system configuration values. name specifies the configuration value to retrieve; it may be a string which is the name of a defined system value; these names are specified in a number of standards (POSIX, Unix 95, Unix 98, and others). Some platforms define additional names as well. The names known to the host operating system are given as the keys of the `confstr_names` dictionary. For configuration variables not included in that mapping, passing an integer for name is also accepted.

  If the configuration value specified by name isn’t defined, `None` is returned.

  If name is a string and is not known, `ValueError` is raised. If a specific value for name is not supported by the host system, even if it is included in `confstr_names`, an `OSError` is raised with `errno.EINVAL` for the error number.

  Availability: Unix.

`os.``confstr_names`

  Dictionary mapping names accepted by `confstr()` to the integer values defined for those names by the host operating system. This can be used to determine the set of names known to the system.

  Availability: Unix.

`os.``cpu_count`()

  Return the number of CPUs in the system. Returns `None` if undetermined.

  This number is not equivalent to the number of CPUs the current process can use. The number of usable CPUs can be obtained with `len(os.sched_getaffinity(0))`

  New in version 3.4.

`os.``getloadavg`()

  Return the number of processes in the system run queue averaged over the last 1, 5, and 15 minutes or raises `OSError` if the load average was unobtainable.

  Availability: Unix.

`os.``sysconf`(name)

  Return integer-valued system configuration values. If the configuration value specified by name isn’t defined, `-1` is returned. The comments regarding the name parameter for `confstr()` apply here as well; the dictionary that provides information on the known names is given by `sysconf_names`.

  Availability: Unix.

`os.``sysconf_names`

  Dictionary mapping names accepted by `sysconf()` to the integer values defined for those names by the host operating system. This can be used to determine the set of names known to the system.

  Availability: Unix.

The following data values are used to support path manipulation operations. These are defined for all platforms.

Higher-level operations on pathnames are defined in the `os.path` module.

`os.``curdir`

  The constant string used by the operating system to refer to the current directory. This is `'.'` for Windows and POSIX. Also available via `os.path`.

`os.``pardir`

  The constant string used by the operating system to refer to the parent directory. This is `'..'` for Windows and POSIX. Also available via `os.path`.

`os.``sep`

  The character used by the operating system to separate pathname components. This is `'/'` for POSIX and `'\\'` for Windows. Note that knowing this is not sufficient to be able to parse or concatenate pathnames — use `os.path.split()` and `os.path.join()` — but it is occasionally useful. Also available via `os.path`.

`os.``altsep`

  An alternative character used by the operating system to separate pathname components, or `None` if only one separator character exists. This is set to `'/'` on Windows systems where `sep` is a backslash. Also available via `os.path`.

`os.``extsep`

  The character which separates the base filename from the extension; for example, the `'.'` in `os.py`. Also available via `os.path`.

`os.``pathsep`

  The character conventionally used by the operating system to separate search path components (as in `PATH`), such as `':'` for POSIX or `';'` for Windows. Also available via `os.path`.

`os.``defpath`

  The default search path used by `exec*p*` and `spawn*p*` if the environment doesn’t have a `'PATH'` key. Also available via `os.path`.

`os.``linesep`

  The string used to separate (or, rather, terminate) lines on the current platform. This may be a single character, such as `'\n'` for POSIX, or multiple characters, for example, `'\r\n'` for Windows. Do not use os.linesep as a line terminator when writing files opened in text mode (the default); use a single `'\n'` instead, on all platforms.

`os.``devnull`

  The file path of the null device. For example: `'/dev/null'` for POSIX, `'nul'` for Windows. Also available via `os.path`.

`os.``RTLD_LAZY`
`os.``RTLD_NOW`
`os.``RTLD_GLOBAL`
`os.``RTLD_LOCAL`
`os.``RTLD_NODELETE`
`os.``RTLD_NOLOAD`
`os.``RTLD_DEEPBIND`

  Flags for use with the `setdlopenflags()` and `getdlopenflags()` functions. See the Unix manual page dlopen(3) for what the different flags mean.

  New in version 3.3.

## Random numbers

`os.``getrandom`(size, flags=0)

  Get up to size random bytes. The function can return less bytes than requested.

  These bytes can be used to seed user-space random number generators or for cryptographic purposes.

  `getrandom()` relies on entropy gathered from device drivers and other sources of environmental noise. Unnecessarily reading large quantities of data will have a negative impact on other users of the `/dev/random` and `/dev/urandom` devices.

  The flags argument is a bit mask that can contain zero or more of the following values ORed together: `os.GRND_RANDOM` and `GRND_NONBLOCK`.

  See also the Linux getrandom() manual page.

  Availability: Linux 3.17 and newer.

  New in version 3.6.

`os.``urandom`(size)

  Return a bytestring of size random bytes suitable for cryptographic use.

  This function returns random bytes from an OS-specific randomness source. The returned data should be unpredictable enough for cryptographic applications, though its exact quality depends on the OS implementation.

  On Linux, if the `getrandom()` syscall is available, it is used in blocking mode: block until the system urandom entropy pool is initialized (128 bits of entropy are collected by the kernel). See the PEP 524 for the rationale. On Linux, the `getrandom()` function can be used to get random bytes in non-blocking mode (using the `GRND_NONBLOCK` flag) or to poll until the system urandom entropy pool is initialized.

  On a Unix-like system, random bytes are read from the `/dev/urandom` device. If the `/dev/urandom` device is not available or not readable, the `NotImplementedError` exception is raised.

  On Windows, it will use `CryptGenRandom()`.

  See also

  The `secrets` module provides higher level functions. For an easy-to-use interface to the random number generator provided by your platform, please see `random.SystemRandom`.

  Changed in version 3.6.0: On Linux, `getrandom()` is now used in blocking mode to increase the security.

  Changed in version 3.5.2: On Linux, if the `getrandom()` syscall blocks (the urandom entropy pool is not initialized yet), fall back on reading `/dev/urandom`.

  Changed in version 3.5: On Linux 3.17 and newer, the `getrandom()` syscall is now used when available. On OpenBSD 5.6 and newer, the C `getentropy()` function is now used. These functions avoid the usage of an internal file descriptor.

`os.``GRND_NONBLOCK`

  By default, when reading from `/dev/random`, `getrandom()` blocks if no random bytes are available, and when reading from `/dev/urandom`, it blocks if the entropy pool has not yet been initialized.

  If the `GRND_NONBLOCK` flag is set, then `getrandom()` does not block in these cases, but instead immediately raises `BlockingIOError`.

  New in version 3.6.

`os.``GRND_RANDOM`

  If this bit is set, then random bytes are drawn from the `/dev/random` pool instead of the `/dev/urandom` pool.

  New in version 3.6.