"os" --- 操作系统接口模块
*************************

**源代码：** Lib/os.py

======================================================================

该模块提供了一些方便使用操作系统相关功能的函数。 如果你是想读写一个文
件，请参阅 "open()"，如果你想操作路径，请参阅 "os.path" 模块，如果你想
在命令行上读取所有文件中的所有行请参阅 "fileinput" 模块。 有关创建临时
文件和目录的方法，请参阅 "tempfile" 模块，对于高级文件目录处理，请参阅
"shutil" 模块。

关于这些函数的可用性的说明：

* 所有 Python 内建的操作系统相关的模块的设计都是为了使得在同一功能可
  用 的情况下，保持接口的一致性；例如，函数 "os.stat(path)" 以相同的格
  式 返回关于 *path* 的统计信息（这个函数同时也是起源于 POSIX 接口）。

* 针对特定的操作的拓展同样在可用于 "os" 模块，但是使用它们必然会对可
  移 植性产生威胁。

* 所有接受路径或文件名的函数都同时支持字节串和字符串对象，并在返回路
  径 或文件名时使用相应类型的对象作为结果。

* 在 VxWorks 系统上，os.fork, os.execv 和 os.spawn*p* 都未支持。

注解: 如果使用无效或无法访问的文件名与路径，或者其他类型正确但操作系
  统不接 受的参数，此模块的所有函数都抛出 "OSError" （或者它的子类）。

exception os.error

   内建的 "OSError" 异常的一个别名。

os.name

   导入的依赖特定操作系统的模块的名称。以下名称目前已注册: "'posix'",
   "'nt'", "'java'".

   参见: "sys.platform" 有更详细的描述. "os.uname()" 只给出系统提供
     的版本 信息。

     "platform" 模块对系统的标识有更详细的检查。


文件名，命令行参数，以及环境变量。
==================================

在 Python 中，使用字符串类型表示文件名、命令行参数和环境变量。 在某些
系统上，在将这些字符串传递给操作系统之前，必须将这些字符串解码为字节。
Python 使用文件系统编码来执行此转换（请参阅
"sys.getfilesystemencoding()" ）。

在 3.1 版更改: 在某些系统上，使用文件系统编码进行转换可能会失败。 在这
种情况下，Python 会使用 代理转义编码错误处理器，这意味着在解码时，不可
解码的字节被 Unicode 字符 U+DCxx 替换，并且这些字节在编码时再次转换为
原始字节。

文件系统编码必须保证成功解码小于 128 的所有字节。如果文件系统编码无法
提供此保证， API 函数可能会引发 UnicodeErrors 。


进程参数
========

这些函数和数据项提供了操作当前进程和用户的信息。

os.ctermid()

   返回与进程控制终端对应的文件名。

   可用性: Unix。

os.environ

   一个表示字符串环境的 *mapping* 对象。 例如，"environ['HOME']" 是你
   的主目录（在某些平台上）的路径名，相当于 C 中的 "getenv("HOME")"。

   这个映射是在第一次导入 "os" 模块时捕获的，通常作为 Python 启动时处
   理 "site.py" 的一部分。除了通过直接修改 "os.environ" 之外，在此之后
   对环境所做的更改不会反映在 "os.environ" 中。

   如果平台支持 "putenv()" 函数，这个映射除了可以用于查询环境外还能用
   于修改环境。 当这个映射被修改时，"putenv()" 将被自动调用。

   在Unix系统上，键和值会使用 "sys.getfilesystemencoding()" 和
   "'surrogateescape'" 的错误处理。如果你想使用其他的编码，使用
   "environb"。

   注解: 直接调用 "putenv()" 并不会影响  "os.environ"，所以推荐直接
     修改 ``os.environ``。

   注解: 在某些平台上，包括 FreeBSD 和 Mac OS X，设置 "environ" 可能
     导致内 存泄露。参阅 "putenv()" 的系统文档。

   如果平台没有提供 "putenv()", 为了使启动的子进程使用修改后的环境，一
   份修改后的映射会被传给合适的进程创建函数。

   如果平台支持 "unsetenv()" 函数，你可以通过删除映射中元素的方式来删
   除对应的环境变量。当一个元素被从 "os.environ" 删除时，以及 "pop()"
   或 "clear()" 被调用时， "unsetenv()" 会被自动调用。

os.environb

   字节版本的 "environ": 一个以字节串表示环境的 *mapping* 对象。
   "environ" 和 "environb" 是同步的（修改 "environb" 会更新 "environ"
   ，反之亦然）。

   只有在 "supports_bytes_environ" 为 True 的时候 "environb" 才是可用
   的。

   3.2 新版功能.

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

   以上函数请参阅 Files and Directories 。

os.fsencode(filename)

   编码 *路径类* *文件名* 为文件系统接受的形式，使用
   "'surrogateescape'" 代理转义编码错误处理器，在Windows系统上会使用
   "'strict'" ；返回 "bytes" 字节类型不变。

   "fsdecode()" 是此函数的逆向函数。

   3.2 新版功能.

   在 3.6 版更改: 增加对实现了 "os.PathLike" 接口的对象的支持。

os.fsdecode(filename)

   从文件系统编码方式解码为 *路径类* 文件名，使用 "'surrogateescape'"
   代理转义编码错误处理器，在Windows系统上会使用 "'strict'" ；返回
   "str" 字符串不变。

   "fsencode()" 是此函数的逆向函数。

   3.2 新版功能.

   在 3.6 版更改: 增加对实现了 "os.PathLike" 接口的对象的支持。

os.fspath(path)

   返回路径的文件系统表示。

   如果传入的是 "str" 或 "bytes" 类型的字符串，将原样返回。否则
   "__fspath__()" 将被调用，如果得到的是一个 "str" 或 "bytes" 类型的对
   象，那就返回这个值。其他所有情况则会抛出 "TypeError"  异常。

   3.6 新版功能.

class os.PathLike

   描述表示一个文件系统路径的 *abstract base class* ，如
   "pathlib.PurePath" 。

   3.6 新版功能.

   abstractmethod __fspath__()

      返回当前对象的文件系统表示。

      这个方法只应该返回一个 "str" 字符串或 "bytes" 字节串，请优先选择
      "str" 字符串。

os.getenv(key, default=None)

   如果存在，返回环境变量 *key* 的值，否则返回 *default*。 *key* ，
   *default* 和返回值均为 str 字符串类型。

   在Unix系统上，键和值会使用 "sys.getfilesystemencoding()" 和
   ``'surrogateescape'`` 错误处理进行解码。如果你想使用其他的编码，使
   用 "os.getenvb()"。

   可用性: 大部分的Unix系统，Windows。

os.getenvb(key, default=None)

   如果存在环境变量 *key* 那么返回其值，否则返回 *default*。 *key* ，
   *default* 和返回值均为bytes字节串类型。

   "getenvb()" 仅在 "supports_bytes_environ" 为 True 时可用

   可用性: 大部分的Unix系统。

   3.2 新版功能.

os.get_exec_path(env=None)

   返回将用于搜索可执行文件的目录列表，与在外壳程序中启动一个进程时相
   似。指定的 *env* 应为用于搜索 PATH 的环境变量字典。默认情况下，当
   *env* 为 "None" 时，将会使用 "environ" 。

   3.2 新版功能.

os.getegid()

   返回当前进程的有效组ID。对应当前进程执行文件的 "set id" 位。

   可用性: Unix。

os.geteuid()

   返回当前进程的有效用户ID。

   可用性: Unix。

os.getgid()

   返回当前进程的实际组ID。

   可用性: Unix。

os.getgrouplist(user, group)

   返回该用户所在的组 ID 列表。可能 *group* 参数没有在返回的列表中，实
   际上用户应该也是属于该 *group*。*group* 参数一般可以从储存账户信息
   的密码记录文件中找到。

   可用性: Unix。

   3.3 新版功能.

os.getgroups()

   返回当前进程对应的组ID列表

   可用性: Unix。

   注解: 在Mac OS X系统中，"getgroups()" 会和其他 Unix 平台有些不同
     。如果 Python 解释器是在 "10.5" 或更早版本中部署，"getgroups()"
     返回当前 用户进程相关的有效组ID列表。 该列表长度由于系统预设的接
     口限制，最 长为 16。 而且在适当的权限下，返回结果还会因
     "getgroups()" 而发生 变化；如果 Python 解释器是在 "10.5" 以上版本
     中部署，"getgroups()" 返回进程所属有效用户 ID 所对应的用户的组 ID
     列表，组用户列表可能 因为进程的生存周期而发生变动，而且也不会因为
     "setgroups()" 的调用 而发生，返回的组用户列表长度也没有长度 16 的
     限制。在部署中， Python 解释器用到的变量
     "MACOSX_DEPLOYMENT_TARGET" 可以用 "sysconfig.get_config_var()"。

os.getlogin()

   返回通过控制终端进程进行登录的用户名。在多数情况下，使用
   "getpass.getuser()" 会更有效，因为后者会通过检查环境变量 "LOGNAME"
   或 "USERNAME" 来查找用户，再由 "pwd.getpwuid(os.getuid())[0]" 来获
   取当前用户 ID 的登录名。

   可用性: Unix, Windows。

os.getpgid(pid)

   根据进程id *pid* 返回进程的组 ID 列表。如果 *pid* 为 0，则返回当前
   进程的进程组 ID 列表

   可用性: Unix。

os.getpgrp()

   返回当时进程组的ID

   可用性: Unix。

os.getpid()

   返回当前进程ID

os.getppid()

   返回父进程ID。当父进程已经结束，在Unix中返回的ID是初始进程(1)中的一
   个，在Windows中仍然是同一个进程ID，该进程ID有可能已经被进行进程所占
   用。

   可用性: Unix, Windows。

   在 3.2 版更改: 添加WIndows的支持。

os.getpriority(which, who)

   获取程序调度优先级。*which* 参数值可以是 "PRIO_PROCESS"，
   "PRIO_PGRP"，或 "PRIO_USER" 中的一个，*who* 是相对于 *which*
   ("PRIO_PROCESS" 的进程标识符，"PRIO_PGRP" 的进程组标识符和
   "PRIO_USER" 的用户ID)。当 *who* 为 0 时（分别）表示调用的进程，调用
   进程的进程组或调用进程所属的真实用户 ID。

   可用性: Unix。

   3.3 新版功能.

os.PRIO_PROCESS
os.PRIO_PGRP
os.PRIO_USER

   函数 "getpriority()" 和 "setpriority()" 的参数。

   可用性: Unix。

   3.3 新版功能.

os.getresuid()

   返回一个由 (ruid, euid, suid) 所组成的元组，分别表示当前进程的真实
   用户ID，有效用户ID和甲暂存用户ID。

   可用性: Unix。

   3.2 新版功能.

os.getresgid()

   返回一个由 (rgid, egid, sgid) 所组成的元组，分别表示当前进程的真实
   组ID，有效组ID和暂存组ID。

   可用性: Unix。

   3.2 新版功能.

os.getuid()

   返回当前进程的真实用户ID。

   可用性: Unix。

os.initgroups(username, gid)

   调用系统 initgroups()，使用指定用户所在的所有值来初始化组访问列表，
   包括指定的组ID。

   可用性: Unix。

   3.2 新版功能.

os.putenv(key, value)

   将名为 *key* 的环境变量值设置为 *value*。该变量名修改会影响由
   "os.system()"， "popen()" ，"fork()" 和 "execv()" 发起的子进程。

   可用性: 大部分的Unix系统，Windows。

   注解: 在一些平台，包括 FreeBSD 和 Mac OS X，设置 "environ" 可能导
     致内存 泄露。详情参考 putenv 相关系统文档。

   当系统支持 "putenv()" 时，"os.environ" 中的参数赋值会自动转换为对
   "putenv()" 的调用。不过 "putenv()" 的调用不会更新 "os.environ"，因
   此最好使用 "os.environ" 对变量赋值。

os.setegid(egid)

   设置当前进程的有效组ID。

   可用性: Unix。

os.seteuid(euid)

   设置当前进程的有效用户ID。

   可用性: Unix。

os.setgid(gid)

   设置当前进程的组ID。

   可用性: Unix。

os.setgroups(groups)

   将 *group* 参数值设置为与当进程相关联的附加组ID列表。*group* 参数必
   须为一个序列，每个元素应为每个组的数字ID。该操作通常只适用于超级用
   户。

   可用性: Unix。

   注解: 在 Mac OS X 中，*groups* 的长度不能超过系统定义的最大有效组
     ID 个 数，一般为 16。 如果它没有返回与调用 setgroups() 所设置的相
     同的组 列表，请参阅 "getgroups()" 的文档。

os.setpgrp()

   根据已实现的版本（如果有）来调用系统 "setpgrp()" 或 "setpgrp(0, 0)"
   。相关说明，请参考 Unix 手册。

   可用性: Unix。

os.setpgid(pid, pgrp)

   使用系统调用 "setpgid()"，将 *pid* 对应进程的组ID设置为 *pgrp*。相
   关说明，请参考 Unix 手册。

   可用性: Unix。

os.setpriority(which, who, priority)

   设置程序调度优先级。 *which* 的值为 "PRIO_PROCESS", "PRIO_PGRP" 或
   "PRIO_USER" 之一，而 *who* 会相对于 *which* ("PRIO_PROCESS" 的进程
   标识符, "PRIO_PGRP" 的进程组标识符和 "PRIO_USER" 的用户 ID) 被解析
   。 *who* 值为零 (分别) 表示调用进程，调用进程的进程组或调用进程的真
   实用户 ID。 *priority* 是范围在 -20 至 19 的值。 默认优先级为 0；较
   小的优先级数值会更优先被调度。

   可用性: Unix。

   3.3 新版功能.

os.setregid(rgid, egid)

   设置当前进程的真实和有效组ID。

   可用性: Unix。

os.setresgid(rgid, egid, sgid)

   设置当前进程的真实，有效和暂存组ID。

   可用性: Unix。

   3.2 新版功能.

os.setresuid(ruid, euid, suid)

   设置当前进程的真实，有效和暂存用户ID。

   可用性: Unix。

   3.2 新版功能.

os.setreuid(ruid, euid)

   设置当前进程的真实和有效用户ID。

   可用性: Unix。

os.getsid(pid)

   调用系统调用 "getsid()"。 相关语义请参阅 Unix 手册。

   可用性: Unix。

os.setsid()

   使用系统调用 "getsid()"。相关说明，请参考 Unix 手册。

   可用性: Unix。

os.setuid(uid)

   设置当前进程的用户ID。

   可用性: Unix。

os.strerror(code)

   根据 *code* 中的错误码返回错误消息。 在某些平台上当给出未知错误码时
   "strerror()" 将返回 "NULL" 并会引发 "ValueError"。

os.supports_bytes_environ

   如果操作系统上原生环境类型是字节型则为 "True" (例如在 Windows 上为
   "False")。

   3.2 新版功能.

os.umask(mask)

   设定当前数值掩码并返回之前的掩码。

os.uname()

   返回当前操作系统的识别信息。返回值是一个有5个属性的对象：

   * "sysname" - 操作系统名

   * "nodename" - 机器在网络上的名称（需要先设定）

   * "release" - 操作系统发行信息

   * "version" - 操作系统版本信息

   * "machine" - 硬件标识符

   为了向后兼容，该对象也是可迭代的，像是一个按照 "sysname"，
   "nodename"，"release"，"version"，和 "machine" 顺序组成的元组。

   有些系统会将 "nodename" 截短为 8 个字符或截短至前缀部分；获取主机名
   的一个更好方式是 "socket.gethostname()"  或甚至可以用
   "socket.gethostbyaddr(socket.gethostname())"。

   可用性: 较新的 Unix 版本。

   在 3.3 版更改: 返回结果的类型由元组变成一个类似元组的对象，同时具有
   命名的属性。

os.unsetenv(key)

   取消设置（删除）名为 *key* 的环境变量。变量名的改变会影响由
   "os.system()"，"popen()"，"fork()" 和 "execv()" 触发的子进程。

   当系统支持 "unsetenv()" ，删除在 "os.environ" 中的变量会自动转换为
   对 "unsetenv()" 的调用。但是 "unsetenv()" 不能更新 "os.environ"，因
   此最好直接删除 "os.environ" 中的变量。

   可用性: 大部分的Unix系统，Windows。


创建文件对象
============

这些函数创建新的 *file objects* 。（参见 "open()" 以获取打开文件描述符
的相关信息。）

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

   返回打开文件描述符 *fd* 对应文件的对象。类似内建 "open()" 函数，二
   者接受同样的参数。不同之处在于 "fdopen()" 第一个参数应该为整数。


文件描述符操作
==============

这些函数对文件描述符所引用的 I/O 流进行操作。

文件描述符是一些小的整数，对应于当前进程所打开的文件。例如，标准输入的
文件描述符通常是0，标准输出是1，标准错误是2。之后被进程打开的文件的文
件描述符会被依次指定为3，4，5等。“文件描述符”这个词有点误导性，在 Unix
平台中套接字和管道也被文件描述符所引用。

当需要时，可以用 "fileno()" 可以获得 *file object* 所对应的文件描述符
。需要注意的是，直接使用文件描述符会绕过文件对象的方法，会忽略如数据内
部缓冲等情况。

os.close(fd)

   关闭文件描述符 *fd*。

   注解: 该功能适用于低级 I/O 操作，必须用于 "os.open()" 或 "pipe()"
     返回 的文件描述符。关闭由内建函数 "open()" ， "popen()" 或
     "fdopen()" 返回的 "文件对象"，则使用其相应的 "close()" 方法。

os.closerange(fd_low, fd_high)

   关闭从 *fd_low* （包括）到 *fd_high* （排除）间的文件描述符，并忽略
   错误。类似（但快于）:

      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)

   从文件描述符 *src* 复制 *count* 字节，从偏移量 *offset_src* 开始读
   取，到文件描述符 *dst*，从偏移量 *offset_dst* 开始写入。如果
   *offset_src* 为 None，则 *src* 将从当前位置开始读取；*offset_dst*
   同理。*src* 和 *dst* 指向的文件必须处于相同的文件系统，否则将会抛出
   一个 "errno" 被设为 "errno.EXDEV" 的 "OSError" 。

   此复制的完成没有额外的从内核到用户空间再回到内核的数据转移花费。另
   外，一些文件系统可能实现额外的优化。完成复制就如同打开两个二进制文
   件一样。

   返回值是复制的字节的数目。这可能低于需求的数目。

   Availability: Linux kernel >= 4.5 或 glibc >= 2.27。

   3.8 新版功能.

os.device_encoding(fd)

   如果连接到终端，则返回一个与 *fd* 关联的设备描述字符，否则返回
   "None"。

os.dup(fd)

   返回一个文件描述符 *fd* 的副本。该文件描述符的副本是 不可继承的。

   在 Windows 中，当复制一个标准流（0: stdin, 1: stdout, 2: stderr）时
   ，新的文件描述符是 可继承的。

   在 3.4 版更改: 新的文件描述符现在是不可继承的。

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

   把文件描述符 *fd* 复制为 *fd2*，必要时先关闭后者。返回 *fd2*。新的
   文件描述符默认是 可继承的，除非在 *inheritable* 为 "False" 时，是不
   可继承的。

   在 3.4 版更改: 添加可选参数 *inheritable*。

   在 3.7 版更改: 成功时返回 *fd2*，以过去的版本中，总是返回 "None"。

os.fchmod(fd, mode)

   将 *fd* 指定文件的权限状态修改为 *mode*。可以参考 "chmod()" 中列出
   *mode* 的可用值。从Python 3.3开始，这相当于 "os.chmod(fd, mode)"。

   可用性: Unix。

os.fchown(fd, uid, gid)

   分别将 *fd* 指定文件的所有者和组 ID 修改为 *uid* 和 *gid* 的值。若
   不想变更其中的某个 ID，可将相应值设为 -1。参考  "chown()"。从
   Python 3.3 开始，这相当于 "os.chown(fd, uid, gid)"。

   可用性: Unix。

os.fdatasync(fd)

   强制将文件描述符 *fd* 指定文件写入磁盘。不强制更新元数据。

   可用性: Unix。

   注解: 该功能在 MacOS 中不可用。

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.

   从 Python 3.3 起，此功能等价于 "os.pathconf(fd, name)"。

   可用性: Unix。

os.fstat(fd)

   获取文件描述符 *fd* 的状态. 返回一个 "stat_result" 对象。

   从 Python 3.3 起，此功能等价于 "os.stat(fd)"。

   参见: "stat()" 函数。

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)".

   可用性: Unix。

os.fsync(fd)

   强制将文件描述符 *fd* 指向的文件写入磁盘。在 Unix，这将调用原生
   "fsync()" 函数；在 Windows，则是 MS "_commit()" 函数。

   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.

   可用性: 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".

   可用性: Unix, Windows。

   在 3.5 版更改: 添加了 Windows 支持

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()".

   可用性: Unix。

   3.5 新版功能.

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.

   可用性: Unix。

   3.3 新版功能.

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

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

   可用性: Unix。

   3.3 新版功能.

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.

   3.3 新版功能: 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".

   在 3.4 版更改: 新的文件描述符现在是不可继承的。

   注解: 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()".

   3.3 新版功能: *dir_fd* 参数。

   在 3.5 版更改: 如果系统调用被中断，但信号处理程序没有触发异常，此函
   数现在会重试系统调用，而不是触发 "InterruptedError" 异常 (原因详见
   **PEP 475**)。

   在 3.6 版更改: 接受一个 *类路径对象*。

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

   这个常数仅在 Unix 系统中可用。

   在 3.3 版更改: 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

   这个常数仅在 Windows 系统中可用。

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.

   在 3.4 版更改: 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.

   可用性: 某些 Unix。

   在 3.4 版更改: 新的文件描述符不再可继承。

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.

   可用性: Unix, Windows。

   在 3.4 版更改: 新的文件描述符不再可继承。

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.

   可用性: 某些 Unix。

   3.3 新版功能.

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.

   可用性: Unix。

   3.3 新版功能.

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".

   可用性: Unix。

   3.3 新版功能.

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.

   可用性: Unix。

   3.3 新版功能.

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.

   可用性: Unix。

   3.3 新版功能.

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. Using flags requires Linux 4.6 or newer.

   3.7 新版功能.

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.

   3.7 新版功能.

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.

   3.7 新版功能.

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.

   可用性: Unix。

   3.3 新版功能.

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. Using flags requires Linux 4.7 or newer.

   3.7 新版功能.

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.

   3.7 新版功能.

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.

   3.7 新版功能.

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.

   注解: 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.

   在 3.5 版更改: 如果系统调用被中断，但信号处理程序没有触发异常，此函
   数现在会重试系统调用，而不是触发 "InterruptedError" 异常 (原因详见
   **PEP 475**)。

os.sendfile(out, in, offset, count)
os.sendfile(out, in, offset, count[, headers][, trailers], flags=0)

   Copy *count* bytes from file descriptor *in* to file descriptor
   *out* 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* and the position of *in* is updated.

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

   On Mac OS X and FreeBSD, a value of 0 for *count* specifies to send
   until the end of *in* is reached.

   All platforms support sockets as *out* 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.

   可用性: Unix。

   注解: For a higher-level wrapper of "sendfile()", see
     "socket.socket.sendfile()".

   3.3 新版功能.

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()".

   可用性: Unix。

   3.5 新版功能.

os.SF_NODISKIO
os.SF_MNOWAIT
os.SF_SYNC

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

   可用性: Unix。

   3.3 新版功能.

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.

   可用性: Unix。

   3.3 新版功能.

os.tcgetpgrp(fd)

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

   可用性: 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*.

   可用性: 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.

   可用性: Unix。

os.write(fd, str)

   Write the bytestring in *str* to file descriptor *fd*.

   Return the number of bytes actually written.

   注解: 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.

   在 3.5 版更改: 如果系统调用被中断，但信号处理程序没有触发异常，此函
   数现在会重试系统调用，而不是触发 "InterruptedError" 异常 (原因详见
   **PEP 475**)。

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.

   可用性: Unix。

   3.3 新版功能.


Querying the size of a terminal
-------------------------------

3.3 新版功能.

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.

   可用性: 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
-------------------------------

3.4 新版功能.

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).

   可用性: Windows。

os.set_handle_inheritable(handle, inheritable)

   Set the "inheritable" flag of the specified handle.

   可用性: 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".

   注解: 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()

   注解: 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.

   在 3.3 版更改: Added the *dir_fd*, *effective_ids*, and
   *follow_symlinks* parameters.

   在 3.6 版更改: 接受一个 *类路径对象*。

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".

   3.3 新版功能: Added support for specifying *path* as a file
   descriptor on some platforms.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: Unix。

   3.3 新版功能: The *follow_symlinks* argument.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   注解: 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.

   3.3 新版功能: Added support for specifying *path* as an open file
   descriptor, and the *dir_fd* and *follow_symlinks* arguments.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: Unix。

   3.3 新版功能: Added support for specifying *path* as an open file
   descriptor, and the *dir_fd* and *follow_symlinks* arguments.

   在 3.6 版更改: Supports a *path-like object*.

os.chroot(path)

   Change the root directory of the current process to *path*.

   可用性: Unix。

   在 3.6 版更改: 接受一个 *类路径对象*。

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)".

   可用性: Unix。

os.getcwd()

   Return a string representing the current working directory.

os.getcwdb()

   Return a bytestring representing the current working directory.

   在 3.8 版更改: 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)".

   可用性: Unix。

   在 3.6 版更改: 接受一个 *类路径对象*。

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)".

   可用性: Unix。

   在 3.6 版更改: 接受一个 *类路径对象*。

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)".

   可用性: Unix。

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: Unix, Windows。

   在 3.2 版更改: Added Windows support.

   3.3 新版功能: Added the *src_dir_fd*, *dst_dir_fd*, and
   *follow_symlinks* arguments.

   在 3.6 版更改: 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.

   *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".

   注解: To encode "str" filenames to "bytes", use "fsencode()".

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

   在 3.2 版更改: The *path* parameter became optional.

   3.3 新版功能: Added support for specifying *path* as an open file
   descriptor.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   参见: "stat()" 函数。

   在 3.2 版更改: Added support for Windows 6.0 (Vista) symbolic
   links.

   在 3.3 版更改: Added the *dir_fd* parameter.

   在 3.6 版更改: Accepts a *path-like object* for *src* and *dst*.

   在 3.8 版更改: 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.

   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.

   3.3 新版功能: *dir_fd* 参数。

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   注解: "makedirs()" will become confused if the path elements to
     create include "pardir" (eg. ".." on UNIX systems).

   This function handles UNC paths correctly.

   3.2 新版功能: The *exist_ok* parameter.

   在 3.4.1 版更改: 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.

   在 3.6 版更改: 接受一个 *类路径对象*。

   在 3.7 版更改: 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.

   可用性: Unix。

   3.3 新版功能: *dir_fd* 参数。

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: Unix。

   3.3 新版功能: *dir_fd* 参数。

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: Unix。

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: 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.

   可用性: Unix, Windows。

   在 3.2 版更改: Added support for Windows 6.0 (Vista) symbolic
   links.

   3.3 新版功能: *dir_fd* 参数。

   在 3.6 版更改: Accepts a *path-like object* on Unix.

   在 3.8 版更改: Accepts a *path-like object* and a bytes object on
   Windows.

   在 3.8 版更改: 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.

   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()".

   3.3 新版功能: *dir_fd* 参数。

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   在 3.6 版更改: 接受一个 *类路径对象*。

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()".

   3.3 新版功能: The *src_dir_fd* and *dst_dir_fd* arguments.

   在 3.6 版更改: 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()".

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

   在 3.6 版更改: 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
   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.

   3.3 新版功能.

   在 3.6 版更改: 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.

   3.3 新版功能: The *dir_fd* parameter.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   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.

      3.6 新版功能.

   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)

   注解: On Unix-based systems, "scandir()" uses the system's
     opendir() and readdir() functions. On Windows, it uses the Win32
     FindFirstFileW and FindNextFileW functions.

   3.5 新版功能.

   3.6 新版功能: 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*.

   在 3.7 版更改: 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.

   3.5 新版功能.

   在 3.6 版更改: 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.

   示例:

      >>> 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

   参见: "fstat()" and "lstat()" functions.

   3.3 新版功能: Added the *dir_fd* and *follow_symlinks* arguments,
   specifying a file descriptor instead of a path.

   在 3.6 版更改: 接受一个 *类路径对象*。

   在 3.8 版更改: 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.

   注解: 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 Mac OS 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.

   3.3 新版功能: Added the "st_atime_ns", "st_mtime_ns", and
   "st_ctime_ns" members.

   3.5 新版功能: Added the "st_file_attributes" member on Windows.

   在 3.5 版更改: Windows now returns the file index as "st_ino" when
   available.

   3.7 新版功能: Added the "st_fstype" member to Solaris/derivatives.

   3.8 新版功能: Added the "st_reparse_tag" member on Windows.

   在 3.8 版更改: 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.

   可用性: Unix。

   在 3.2 版更改: The "ST_RDONLY" and "ST_NOSUID" constants were
   added.

   3.3 新版功能: Added support for specifying *path* as an open file
   descriptor.

   在 3.4 版更改: 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.

   在 3.6 版更改: 接受一个 *类路径对象*。

   3.7 新版功能: 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.

   3.3 新版功能.

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.

   3.3 新版功能.

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

   3.3 新版功能.

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

   3.3 新版功能.

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.

   注解: 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.

   可用性: Unix, Windows。

   在 3.2 版更改: Added support for Windows 6.0 (Vista) symbolic
   links.

   3.3 新版功能: Added the *dir_fd* argument, and now allow
   *target_is_directory* on non-Windows platforms.

   在 3.6 版更改: Accepts a *path-like object* for *src* and *dst*.

   在 3.8 版更改: Added support for unelevated symlinks on Windows
   with Developer Mode.

os.sync()

   Force write of everything to disk.

   可用性: Unix。

   3.3 新版功能.

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".

   可用性: Unix, Windows。

   3.3 新版功能.

   在 3.5 版更改: 添加了 Windows 支持

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   3.3 新版功能: The *dir_fd* parameter.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   3.3 新版功能: Added support for specifying *path* as an open file
   descriptor, and the *dir_fd*, *follow_symlinks*, and *ns*
   parameters.

   在 3.6 版更改: 接受一个 *类路径对象*。

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)".

   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.

   注解: 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.

   注解: 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))

   在 3.5 版更改: This function now calls "os.scandir()" instead of
   "os.listdir()", making it faster by reducing the number of calls to
   "os.stat()".

   在 3.6 版更改: 接受一个 *类路径对象*。

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".

   注解: 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)

   可用性: Unix。

   3.3 新版功能.

   在 3.6 版更改: 接受一个 *类路径对象*。

   在 3.7 版更改: 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.

   3.8 新版功能.

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.

   3.8 新版功能.


Linux extended attributes
-------------------------

3.3 新版功能.

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.

   在 3.6 版更改: 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.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   在 3.6 版更改: 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, "EEXISTS" will be raised. If
   "XATTR_CREATE" is given and the attribute already exists, the
   attribute will not be created and "ENODATA" will be raised.

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

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

   在 3.6 版更改: 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.

   可用性: Windows。

   3.8 新版功能: 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".

   可用性: Unix, Windows。

   3.3 新版功能: Added support for specifying *path* as an open file
   descriptor for "execve()".

   在 3.6 版更改: 接受一个 *类路径对象*。

os._exit(n)

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

   注解: 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.

注解: 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.

   可用性: Unix。

os.EX_USAGE

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

   可用性: Unix。

os.EX_DATAERR

   Exit code that means the input data was incorrect.

   可用性: Unix。

os.EX_NOINPUT

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

   可用性: Unix。

os.EX_NOUSER

   Exit code that means a specified user did not exist.

   可用性: Unix。

os.EX_NOHOST

   Exit code that means a specified host did not exist.

   可用性: Unix。

os.EX_UNAVAILABLE

   Exit code that means that a required service is unavailable.

   可用性: Unix。

os.EX_SOFTWARE

   Exit code that means an internal software error was detected.

   可用性: Unix。

os.EX_OSERR

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

   可用性: 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.

   可用性: Unix。

os.EX_CANTCREAT

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

   可用性: Unix。

os.EX_IOERR

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

   可用性: 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.

   可用性: Unix。

os.EX_PROTOCOL

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

   可用性: Unix。

os.EX_NOPERM

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

   可用性: Unix。

os.EX_CONFIG

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

   可用性: Unix。

os.EX_NOTFOUND

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

   可用性: 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.

   警告: See "ssl" for applications that use the SSL module with
     fork().

   可用性: 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.

   可用性: 某些 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()".

   3.2 新版功能: Windows support.

os.killpg(pgid, sig)

   Send the signal *sig* to the process group *pgid*.

   可用性: Unix。

os.nice(increment)

   Add *increment* to the process's "niceness".  Return the new
   niceness.

   可用性: Unix。

os.plock(op)

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

   可用性: 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.

   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.

   3.8 新版功能.

   可用性: 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)").

   3.8 新版功能.

   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.

   可用性: Unix。

   3.7 新版功能.

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)

   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.

   在 3.6 版更改: 接受一个 *类路径对象*。

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.

   可用性: 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.

   可用性: 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.

   可用性: 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.

   可用性: 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.

   On Unix, the return value is the exit status of the process encoded
   in the format specified for "wait()".  Note that POSIX does not
   specify the meaning of the return value of the C "system()"
   function, so the return value of the Python function is system-
   dependent.

   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.

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

   可用性: 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
   <https://docs.microsoft.com/windows/win32/api/processthreadsapi/nf-
   processthreadsapi-getprocesstimes>* _ on Windows. On Windows, only
   "user" and "system" are known; the other attributes are zero.

   可用性: Unix, Windows。

   在 3.3 版更改: 返回结果的类型由元组变成一个类似元组的对象，同时具有
   命名的属性。

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.

   可用性: Unix。

os.waitid(idtype, id, options)

   Wait for the completion of one or more child processes. *idtype*
   can be "P_PID", "P_PGID" or "P_ALL". *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.

   可用性: Unix。

   3.3 新版功能.

os.P_PID
os.P_PGID
os.P_ALL

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

   可用性: Unix。

   3.3 新版功能.

os.WEXITED
os.WSTOPPED
os.WNOWAIT

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

   可用性: Unix。

   3.3 新版功能.

os.CLD_EXITED
os.CLD_DUMPED
os.CLD_TRAPPED
os.CLD_CONTINUED

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

   可用性: Unix。

   3.3 新版功能.

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.

   在 3.5 版更改: 如果系统调用被中断，但信号处理程序没有触发异常，此函
   数现在会重试系统调用，而不是触发 "InterruptedError" 异常 (原因详见
   **PEP 475**)。

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()".

   可用性: 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()".

   可用性: Unix。

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.

   可用性: 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.

   可用性: 部分 Unix 系统。

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.

   可用性: 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".

   可用性: Unix。

os.WIFCONTINUED(status)

   Return "True" if the process has been continued from a job control
   stop, otherwise return "False".

   可用性: Unix。

os.WIFSTOPPED(status)

   Return "True" if the process has been stopped, otherwise return
   "False".

   可用性: Unix。

os.WIFSIGNALED(status)

   Return "True" if the process exited due to a signal, otherwise
   return "False".

   可用性: Unix。

os.WIFEXITED(status)

   Return "True" if the process exited using the *exit(2)* system
   call, otherwise return "False".

   可用性: Unix。

os.WEXITSTATUS(status)

   如果 "WIFEXITED(status)" 为值，则将整数形参返回给 *exit(2)* 系统调
   用。 否则，返回值将没有任何意义。

   可用性: Unix。

os.WSTOPSIG(status)

   返回导致进程停止的信号。

   可用性: Unix。

os.WTERMSIG(status)

   返回导致进程退出的信号。

   可用性: Unix。


调度器接口
==========

这些函数控制操作系统如何为进程分配 CPU 时间。 它们仅在某些 Unix 平台上
可用。 更多细节信息请查阅你所用 Unix 的指南页面。

3.3 新版功能.

以下调度策略如果被操作系统支持就会对外公开。

os.SCHED_OTHER

   默认调度策略。

os.SCHED_BATCH

   用于 CPU 密集型进程的调度策略，它会尽量为计算机中的其余任务保留交互
   性。

os.SCHED_IDLE

   用于极低优先级的后台任务的调度策略。

os.SCHED_SPORADIC

   用于偶发型服务程序的调度策略。

os.SCHED_FIFO

   先进先出的调度策略。

os.SCHED_RR

   循环式的调度策略。

os.SCHED_RESET_ON_FORK

   此旗标可与任何其他调度策略进行 OR 运算。 当带有此旗标的进程设置分叉
   时，其子进程的调度策略和优先级会被重置为默认值。

class os.sched_param(sched_priority)

   这个类表示在 "sched_setparam()", "sched_setscheduler()" 和
   "sched_getparam()" 中使用的可修改调度形参。 它属于不可变对象。

   目前它只有一个可能的形参：

   sched_priority

      一个调度策略的调度优先级。

os.sched_get_priority_min(policy)

   获取 *policy* 的最小优先级数值。 *policy* 是以上调度策略常量之一。

os.sched_get_priority_max(policy)

   获取 *policy* 的最高优先级数值。 *policy* 是以上调度策略常量之一。

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 a 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.

   可用性: 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.

   可用性: Unix。

os.cpu_count()

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

   该数量不同于当前进程可以使用的CPU数量。可用的CPU数量可以由
   "len(os.sched_getaffinity(0))" 方法获得。

   3.4 新版功能.

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.

   可用性: 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".

   可用性: 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.

   可用性: 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.

   3.3 新版功能.


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.

   3.6 新版功能.

os.urandom(size)

   Return a string 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()".

   参见: 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".

   在 3.6.0 版更改: On Linux, "getrandom()" is now used in blocking
   mode to increase the security.

   在 3.5.2 版更改: On Linux, if the "getrandom()" syscall blocks (the
   urandom entropy pool is not initialized yet), fall back on reading
   "/dev/urandom".

   在 3.5 版更改: 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".

   3.6 新版功能.

os.GRND_RANDOM

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

   3.6 新版功能.
