17.5. "subprocess" --- Subprocess management
********************************************

**Source code:** Lib/subprocess.py

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

The "subprocess" module allows you to spawn new processes, connect to
their input/output/error pipes, and obtain their return codes.  This
module intends to replace several older modules and functions:

   os.system
   os.spawn*

Information about how the "subprocess" module can be used to replace
these modules and functions can be found in the following sections.

也參考: **PEP 324** -- PEP proposing the subprocess module


17.5.1. Using the "subprocess" Module
=====================================

The recommended approach to invoking subprocesses is to use the
"run()" function for all use cases it can handle. For more advanced
use cases, the underlying "Popen" interface can be used directly.

The "run()" function was added in Python 3.5; if you need to retain
compatibility with older versions, see the Older high-level API
section.

subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, check=False, encoding=None, errors=None, env=None)

   Run the command described by *args*.  Wait for command to complete,
   then return a "CompletedProcess" instance.

   The arguments shown above are merely the most common ones,
   described below in Frequently Used Arguments (hence the use of
   keyword-only notation in the abbreviated signature). The full
   function signature is largely the same as that of the "Popen"
   constructor - apart from *timeout*, *input* and *check*, all the
   arguments to this function are passed through to that interface.

   This does not capture stdout or stderr by default. To do so, pass
   "PIPE" for the *stdout* and/or *stderr* arguments.

   The *timeout* argument is passed to "Popen.communicate()". If the
   timeout expires, the child process will be killed and waited for.
   The "TimeoutExpired" exception will be re-raised after the child
   process has terminated.

   The *input* argument is passed to "Popen.communicate()" and thus to
   the subprocess's stdin.  If used it must be a byte sequence, or a
   string if *encoding* or *errors* is specified or
   *universal_newlines* is true.  When used, the internal "Popen"
   object is automatically created with "stdin=PIPE", and the *stdin*
   argument may not be used as well.

   If *check* is true, and the process exits with a non-zero exit
   code, a "CalledProcessError" exception will be raised. Attributes
   of that exception hold the arguments, the exit code, and stdout and
   stderr if they were captured.

   If *encoding* or *errors* are specified, or *universal_newlines* is
   true, file objects for stdin, stdout and stderr are opened in text
   mode using the specified *encoding* and *errors* or the
   "io.TextIOWrapper" default. Otherwise, file objects are opened in
   binary mode.

   If *env* is not "None", it must be a mapping that defines the
   environment variables for the new process; these are used instead
   of the default behavior of inheriting the current process'
   environment. It is passed directly to "Popen".

   Examples:

      >>> subprocess.run(["ls", "-l"])  # doesn't capture output
      CompletedProcess(args=['ls', '-l'], returncode=0)

      >>> subprocess.run("exit 1", shell=True, check=True)
      Traceback (most recent call last):
        ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

      >>> subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
      CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
      stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n')

   3.5 版新加入.

   3.6 版更變: Added *encoding* and *errors* parameters

class subprocess.CompletedProcess

   The return value from "run()", representing a process that has
   finished.

   args

      The arguments used to launch the process. This may be a list or
      a string.

   returncode

      Exit status of the child process. Typically, an exit status of 0
      indicates that it ran successfully.

      A negative value "-N" indicates that the child was terminated by
      signal "N" (POSIX only).

   stdout

      Captured stdout from the child process. A bytes sequence, or a
      string if "run()" was called with an encoding or errors. "None"
      if stdout was not captured.

      If you ran the process with "stderr=subprocess.STDOUT", stdout
      and stderr will be combined in this attribute, and "stderr" will
      be "None".

   stderr

      Captured stderr from the child process. A bytes sequence, or a
      string if "run()" was called with an encoding or errors. "None"
      if stderr was not captured.

   check_returncode()

      If "returncode" is non-zero, raise a "CalledProcessError".

   3.5 版新加入.

subprocess.DEVNULL

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to "Popen" and indicates that the special file
   "os.devnull" will be used.

   3.3 版新加入.

subprocess.PIPE

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to "Popen" and indicates that a pipe to the standard
   stream should be opened.  Most useful with "Popen.communicate()".

subprocess.STDOUT

   Special value that can be used as the *stderr* argument to "Popen"
   and indicates that standard error should go into the same handle as
   standard output.

exception subprocess.SubprocessError

   Base class for all other exceptions from this module.

   3.3 版新加入.

exception subprocess.TimeoutExpired

   Subclass of "SubprocessError", raised when a timeout expires while
   waiting for a child process.

   cmd

      Command that was used to spawn the child process.

   timeout

      Timeout in seconds.

   output

      Output of the child process if it was captured by "run()" or
      "check_output()".  Otherwise, "None".

   stdout

      Alias for output, for symmetry with "stderr".

   stderr

      Stderr output of the child process if it was captured by
      "run()". Otherwise, "None".

   3.3 版新加入.

   3.5 版更變: *stdout* and *stderr* attributes added

exception subprocess.CalledProcessError

   Subclass of "SubprocessError", raised when a process run by
   "check_call()" or "check_output()" returns a non-zero exit status.

   returncode

      Exit status of the child process.  If the process exited due to
      a signal, this will be the negative signal number.

   cmd

      Command that was used to spawn the child process.

   output

      Output of the child process if it was captured by "run()" or
      "check_output()".  Otherwise, "None".

   stdout

      Alias for output, for symmetry with "stderr".

   stderr

      Stderr output of the child process if it was captured by
      "run()". Otherwise, "None".

   3.5 版更變: *stdout* and *stderr* attributes added


17.5.1.1. Frequently Used Arguments
-----------------------------------

To support a wide variety of use cases, the "Popen" constructor (and
the convenience functions) accept a large number of optional
arguments. For most typical use cases, many of these arguments can be
safely left at their default values. The arguments that are most
commonly needed are:

   *args* is required for all calls and should be a string, or a
   sequence of program arguments. Providing a sequence of arguments is
   generally preferred, as it allows the module to take care of any
   required escaping and quoting of arguments (e.g. to permit spaces
   in file names). If passing a single string, either *shell* must be
   "True" (see below) or else the string must simply name the program
   to be executed without specifying any arguments.

   *stdin*, *stdout* and *stderr* specify the executed program's
   standard input, standard output and standard error file handles,
   respectively.  Valid values are "PIPE", "DEVNULL", an existing file
   descriptor (a positive integer), an existing file object, and
   "None".  "PIPE" indicates that a new pipe to the child should be
   created.  "DEVNULL" indicates that the special file "os.devnull"
   will be used.  With the default settings of "None", no redirection
   will occur; the child's file handles will be inherited from the
   parent.  Additionally, *stderr* can be "STDOUT", which indicates
   that the stderr data from the child process should be captured into
   the same file handle as for *stdout*.

   If *encoding* or *errors* are specified, or *universal_newlines* is
   true, the file objects *stdin*, *stdout* and *stderr* will be
   opened in text mode using the *encoding* and *errors* specified in
   the call or the defaults for "io.TextIOWrapper".

   For *stdin*, line ending characters "'\n'" in the input will be
   converted to the default line separator "os.linesep". For *stdout*
   and *stderr*, all line endings in the output will be converted to
   "'\n'".  For more information see the documentation of the
   "io.TextIOWrapper" class when the *newline* argument to its
   constructor is "None".

   If text mode is not used, *stdin*, *stdout* and *stderr* will be
   opened as binary streams. No encoding or line ending conversion is
   performed.

   3.6 版新加入: Added *encoding* and *errors* parameters.

   備註: The newlines attribute of the file objects "Popen.stdin",
     "Popen.stdout" and "Popen.stderr" are not updated by the
     "Popen.communicate()" method.

   If *shell* is "True", the specified command will be executed
   through the shell.  This can be useful if you are using Python
   primarily for the enhanced control flow it offers over most system
   shells and still want convenient access to other shell features
   such as shell pipes, filename wildcards, environment variable
   expansion, and expansion of "~" to a user's home directory.
   However, note that Python itself offers implementations of many
   shell-like features (in particular, "glob", "fnmatch", "os.walk()",
   "os.path.expandvars()", "os.path.expanduser()", and "shutil").

   3.3 版更變: When *universal_newlines* is "True", the class uses the
   encoding "locale.getpreferredencoding(False)" instead of
   "locale.getpreferredencoding()".  See the "io.TextIOWrapper" class
   for more information on this change.

   備註: Read the Security Considerations section before using
     "shell=True".

These options, along with all of the other options, are described in
more detail in the "Popen" constructor documentation.


17.5.1.2. Popen Constructor
---------------------------

The underlying process creation and management in this module is
handled by the "Popen" class. It offers a lot of flexibility so that
developers are able to handle the less common cases not covered by the
convenience functions.

class subprocess.Popen(args, bufsize=-1, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=True, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=(), *, encoding=None, errors=None)

   Execute a child program in a new process.  On POSIX, the class uses
   "os.execvp()"-like behavior to execute the child program.  On
   Windows, the class uses the Windows "CreateProcess()" function.
   The arguments to "Popen" are as follows.

   *args* should be a sequence of program arguments or else a single
   string. By default, the program to execute is the first item in
   *args* if *args* is a sequence.  If *args* is a string, the
   interpretation is platform-dependent and described below.  See the
   *shell* and *executable* arguments for additional differences from
   the default behavior.  Unless otherwise stated, it is recommended
   to pass *args* as a sequence.

   On POSIX, if *args* is a string, the string is interpreted as the
   name or path of the program to execute.  However, this can only be
   done if not passing arguments to the program.

   備註: "shlex.split()" can be useful when determining the correct
     tokenization for *args*, especially in complex cases:

        >>> import shlex, subprocess
        >>> command_line = input()
        /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
        >>> args = shlex.split(command_line)
        >>> print(args)
        ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
        >>> p = subprocess.Popen(args) # Success!

     Note in particular that options (such as *-input*) and arguments
     (such as *eggs.txt*) that are separated by whitespace in the
     shell go in separate list elements, while arguments that need
     quoting or backslash escaping when used in the shell (such as
     filenames containing spaces or the *echo* command shown above)
     are single list elements.

   On Windows, if *args* is a sequence, it will be converted to a
   string in a manner described in Converting an argument sequence to
   a string on Windows.  This is because the underlying
   "CreateProcess()" operates on strings.

   The *shell* argument (which defaults to "False") specifies whether
   to use the shell as the program to execute.  If *shell* is "True",
   it is recommended to pass *args* as a string rather than as a
   sequence.

   On POSIX with "shell=True", the shell defaults to "/bin/sh".  If
   *args* is a string, the string specifies the command to execute
   through the shell.  This means that the string must be formatted
   exactly as it would be when typed at the shell prompt.  This
   includes, for example, quoting or backslash escaping filenames with
   spaces in them.  If *args* is a sequence, the first item specifies
   the command string, and any additional items will be treated as
   additional arguments to the shell itself.  That is to say, "Popen"
   does the equivalent of:

      Popen(['/bin/sh', '-c', args[0], args[1], ...])

   On Windows with "shell=True", the "COMSPEC" environment variable
   specifies the default shell.  The only time you need to specify
   "shell=True" on Windows is when the command you wish to execute is
   built into the shell (e.g. **dir** or **copy**).  You do not need
   "shell=True" to run a batch file or console-based executable.

   備註: Read the Security Considerations section before using
     "shell=True".

   *bufsize* will be supplied as the corresponding argument to the
   "open()" function when creating the stdin/stdout/stderr pipe file
   objects:

   * "0" means unbuffered (read and write are one system call and
     can return short)

   * "1" means line buffered (only usable if
     "universal_newlines=True" i.e., in a text mode)

   * any other positive value means use a buffer of approximately
     that size

   * negative bufsize (the default) means the system default of
     io.DEFAULT_BUFFER_SIZE will be used.

   3.3.1 版更變: *bufsize* now defaults to -1 to enable buffering by
   default to match the behavior that most code expects.  In versions
   prior to Python 3.2.4 and 3.3.1 it incorrectly defaulted to "0"
   which was unbuffered and allowed short reads.  This was
   unintentional and did not match the behavior of Python 2 as most
   code expected.

   The *executable* argument specifies a replacement program to
   execute.   It is very seldom needed.  When "shell=False",
   *executable* replaces the program to execute specified by *args*.
   However, the original *args* is still passed to the program.  Most
   programs treat the program specified by *args* as the command name,
   which can then be different from the program actually executed.  On
   POSIX, the *args* name becomes the display name for the executable
   in utilities such as **ps**.  If "shell=True", on POSIX the
   *executable* argument specifies a replacement shell for the default
   "/bin/sh".

   *stdin*, *stdout* and *stderr* specify the executed program's
   standard input, standard output and standard error file handles,
   respectively.  Valid values are "PIPE", "DEVNULL", an existing file
   descriptor (a positive integer), an existing *file object*, and
   "None".  "PIPE" indicates that a new pipe to the child should be
   created.  "DEVNULL" indicates that the special file "os.devnull"
   will be used. With the default settings of "None", no redirection
   will occur; the child's file handles will be inherited from the
   parent.  Additionally, *stderr* can be "STDOUT", which indicates
   that the stderr data from the applications should be captured into
   the same file handle as for stdout.

   If *preexec_fn* is set to a callable object, this object will be
   called in the child process just before the child is executed.
   (POSIX only)

   警告: The *preexec_fn* parameter is not safe to use in the
     presence of threads in your application.  The child process could
     deadlock before exec is called. If you must use it, keep it
     trivial! Minimize the number of libraries you call into.

   備註: If you need to modify the environment for the child use the
     *env* parameter rather than doing it in a *preexec_fn*. The
     *start_new_session* parameter can take the place of a previously
     common use of *preexec_fn* to call os.setsid() in the child.

   If *close_fds* is true, all file descriptors except "0", "1" and
   "2" will be closed before the child process is executed. (POSIX
   only). The default varies by platform:  Always true on POSIX.  On
   Windows it is true when *stdin*/*stdout*/*stderr* are "None", false
   otherwise. On Windows, if *close_fds* is true then no handles will
   be inherited by the child process.  Note that on Windows, you
   cannot set *close_fds* to true and also redirect the standard
   handles by setting *stdin*, *stdout* or *stderr*.

   3.2 版更變: The default for *close_fds* was changed from "False" to
   what is described above.

   *pass_fds* is an optional sequence of file descriptors to keep open
   between the parent and child.  Providing any *pass_fds* forces
   *close_fds* to be "True".  (POSIX only)

   3.2 版新加入: The *pass_fds* parameter was added.

   If *cwd* is not "None", the function changes the working directory
   to *cwd* before executing the child.  *cwd* can be a "str" and
   *path-like* object.  In particular, the function looks for
   *executable* (or for the first item in *args*) relative to *cwd* if
   the executable path is a relative path.

   3.6 版更變: *cwd* parameter accepts a *path-like object*.

   If *restore_signals* is true (the default) all signals that Python
   has set to SIG_IGN are restored to SIG_DFL in the child process
   before the exec. Currently this includes the SIGPIPE, SIGXFZ and
   SIGXFSZ signals. (POSIX only)

   3.2 版更變: *restore_signals* was added.

   If *start_new_session* is true the setsid() system call will be
   made in the child process prior to the execution of the subprocess.
   (POSIX only)

   3.2 版更變: *start_new_session* was added.

   If *env* is not "None", it must be a mapping that defines the
   environment variables for the new process; these are used instead
   of the default behavior of inheriting the current process'
   environment.

   備註: If specified, *env* must provide any variables required for
     the program to execute.  On Windows, in order to run a side-by-
     side assembly the specified *env* **must** include a valid
     "SystemRoot".

   If *encoding* or *errors* are specified, the file objects *stdin*,
   *stdout* and *stderr* are opened in text mode with the specified
   encoding and *errors*, as described above in Frequently Used
   Arguments. If *universal_newlines* is "True", they are opened in
   text mode with default encoding. Otherwise, they are opened as
   binary streams.

   3.6 版新加入: *encoding* and *errors* were added.

   If given, *startupinfo* will be a "STARTUPINFO" object, which is
   passed to the underlying "CreateProcess" function. *creationflags*,
   if given, can be "CREATE_NEW_CONSOLE" or
   "CREATE_NEW_PROCESS_GROUP". (Windows only)

   Popen objects are supported as context managers via the "with"
   statement: on exit, standard file descriptors are closed, and the
   process is waited for.

      with Popen(["ifconfig"], stdout=PIPE) as proc:
          log.write(proc.stdout.read())

   3.2 版更變: Added context manager support.

   3.6 版更變: Popen destructor now emits a "ResourceWarning" warning
   if the child process is still running.


17.5.1.3. Exceptions
--------------------

Exceptions raised in the child process, before the new program has
started to execute, will be re-raised in the parent.  Additionally,
the exception object will have one extra attribute called
"child_traceback", which is a string containing traceback information
from the child's point of view.

The most common exception raised is "OSError".  This occurs, for
example, when trying to execute a non-existent file.  Applications
should prepare for "OSError" exceptions.

A "ValueError" will be raised if "Popen" is called with invalid
arguments.

"check_call()" and "check_output()" will raise "CalledProcessError" if
the called process returns a non-zero return code.

All of the functions and methods that accept a *timeout* parameter,
such as "call()" and "Popen.communicate()" will raise "TimeoutExpired"
if the timeout expires before the process exits.

Exceptions defined in this module all inherit from "SubprocessError".

   3.3 版新加入: The "SubprocessError" base class was added.


17.5.2. Security Considerations
===============================

Unlike some other popen functions, this implementation will never
implicitly call a system shell.  This means that all characters,
including shell metacharacters, can safely be passed to child
processes. If the shell is invoked explicitly, via "shell=True", it is
the application's responsibility to ensure that all whitespace and
metacharacters are quoted appropriately to avoid shell injection
vulnerabilities.

When using "shell=True", the "shlex.quote()" function can be used to
properly escape whitespace and shell metacharacters in strings that
are going to be used to construct shell commands.


17.5.3. Popen Objects
=====================

Instances of the "Popen" class have the following methods:

Popen.poll()

   Check if child process has terminated.  Set and return "returncode"
   attribute. Otherwise, returns "None".

Popen.wait(timeout=None)

   Wait for child process to terminate.  Set and return "returncode"
   attribute.

   If the process does not terminate after *timeout* seconds, raise a
   "TimeoutExpired" exception.  It is safe to catch this exception and
   retry the wait.

   備註: This will deadlock when using "stdout=PIPE" or
     "stderr=PIPE" and the child process generates enough output to a
     pipe such that it blocks waiting for the OS pipe buffer to accept
     more data. Use "Popen.communicate()" when using pipes to avoid
     that.

   備註: The function is implemented using a busy loop (non-blocking
     call and short sleeps). Use the "asyncio" module for an
     asynchronous wait: see "asyncio.create_subprocess_exec".

   3.3 版更變: *timeout* was added.

   3.4 版後已棄用: Do not use the *endtime* parameter.  It is was
   unintentionally exposed in 3.3 but was left undocumented as it was
   intended to be private for internal use.  Use *timeout* instead.

Popen.communicate(input=None, timeout=None)

   Interact with process: Send data to stdin.  Read data from stdout
   and stderr, until end-of-file is reached.  Wait for process to
   terminate.  The optional *input* argument should be data to be sent
   to the child process, or "None", if no data should be sent to the
   child.  If streams were opened in text mode, *input* must be a
   string.  Otherwise, it must be bytes.

   "communicate()" returns a tuple "(stdout_data, stderr_data)". The
   data will be strings if streams were opened in text mode;
   otherwise, bytes.

   Note that if you want to send data to the process's stdin, you need
   to create the Popen object with "stdin=PIPE".  Similarly, to get
   anything other than "None" in the result tuple, you need to give
   "stdout=PIPE" and/or "stderr=PIPE" too.

   If the process does not terminate after *timeout* seconds, a
   "TimeoutExpired" exception will be raised.  Catching this exception
   and retrying communication will not lose any output.

   The child process is not killed if the timeout expires, so in order
   to cleanup properly a well-behaved application should kill the
   child process and finish communication:

      proc = subprocess.Popen(...)
      try:
          outs, errs = proc.communicate(timeout=15)
      except TimeoutExpired:
          proc.kill()
          outs, errs = proc.communicate()

   備註: The data read is buffered in memory, so do not use this
     method if the data size is large or unlimited.

   3.3 版更變: *timeout* was added.

Popen.send_signal(signal)

   Sends the signal *signal* to the child.

   備註: On Windows, SIGTERM is an alias for "terminate()".
     CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes
     started with a *creationflags* parameter which includes
     *CREATE_NEW_PROCESS_GROUP*.

Popen.terminate()

   Stop the child. On Posix OSs the method sends SIGTERM to the child.
   On Windows the Win32 API function "TerminateProcess()" is called to
   stop the child.

Popen.kill()

   Kills the child. On Posix OSs the function sends SIGKILL to the
   child. On Windows "kill()" is an alias for "terminate()".

The following attributes are also available:

Popen.args

   The *args* argument as it was passed to "Popen" -- a sequence of
   program arguments or else a single string.

   3.3 版新加入.

Popen.stdin

   If the *stdin* argument was "PIPE", this attribute is a writeable
   stream object as returned by "open()". If the *encoding* or
   *errors* arguments were specified or the *universal_newlines*
   argument was "True", the stream is a text stream, otherwise it is a
   byte stream. If the *stdin* argument was not "PIPE", this attribute
   is "None".

Popen.stdout

   If the *stdout* argument was "PIPE", this attribute is a readable
   stream object as returned by "open()". Reading from the stream
   provides output from the child process. If the *encoding* or
   *errors* arguments were specified or the *universal_newlines*
   argument was "True", the stream is a text stream, otherwise it is a
   byte stream. If the *stdout* argument was not "PIPE", this
   attribute is "None".

Popen.stderr

   If the *stderr* argument was "PIPE", this attribute is a readable
   stream object as returned by "open()". Reading from the stream
   provides error output from the child process. If the *encoding* or
   *errors* arguments were specified or the *universal_newlines*
   argument was "True", the stream is a text stream, otherwise it is a
   byte stream. If the *stderr* argument was not "PIPE", this
   attribute is "None".

警告: Use "communicate()" rather than ".stdin.write", ".stdout.read"
  or ".stderr.read" to avoid deadlocks due to any of the other OS pipe
  buffers filling up and blocking the child process.

Popen.pid

   The process ID of the child process.

   Note that if you set the *shell* argument to "True", this is the
   process ID of the spawned shell.

Popen.returncode

   The child return code, set by "poll()" and "wait()" (and indirectly
   by "communicate()").  A "None" value indicates that the process
   hasn't terminated yet.

   A negative value "-N" indicates that the child was terminated by
   signal "N" (POSIX only).


17.5.4. Windows Popen Helpers
=============================

The "STARTUPINFO" class and following constants are only available on
Windows.

class subprocess.STARTUPINFO

   Partial support of the Windows STARTUPINFO structure is used for
   "Popen" creation.

   dwFlags

      A bit field that determines whether certain "STARTUPINFO"
      attributes are used when the process creates a window.

         si = subprocess.STARTUPINFO()
         si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW

   hStdInput

      If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
      the standard input handle for the process. If
      "STARTF_USESTDHANDLES" is not specified, the default for
      standard input is the keyboard buffer.

   hStdOutput

      If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
      the standard output handle for the process. Otherwise, this
      attribute is ignored and the default for standard output is the
      console window's buffer.

   hStdError

      If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
      the standard error handle for the process. Otherwise, this
      attribute is ignored and the default for standard error is the
      console window's buffer.

   wShowWindow

      If "dwFlags" specifies "STARTF_USESHOWWINDOW", this attribute
      can be any of the values that can be specified in the "nCmdShow"
      parameter for the ShowWindow function, except for
      "SW_SHOWDEFAULT". Otherwise, this attribute is ignored.

      "SW_HIDE" is provided for this attribute. It is used when
      "Popen" is called with "shell=True".


17.5.4.1. Constants
-------------------

The "subprocess" module exposes the following constants.

subprocess.STD_INPUT_HANDLE

   The standard input device. Initially, this is the console input
   buffer, "CONIN$".

subprocess.STD_OUTPUT_HANDLE

   The standard output device. Initially, this is the active console
   screen buffer, "CONOUT$".

subprocess.STD_ERROR_HANDLE

   The standard error device. Initially, this is the active console
   screen buffer, "CONOUT$".

subprocess.SW_HIDE

   Hides the window. Another window will be activated.

subprocess.STARTF_USESTDHANDLES

   Specifies that the "STARTUPINFO.hStdInput",
   "STARTUPINFO.hStdOutput", and "STARTUPINFO.hStdError" attributes
   contain additional information.

subprocess.STARTF_USESHOWWINDOW

   Specifies that the "STARTUPINFO.wShowWindow" attribute contains
   additional information.

subprocess.CREATE_NEW_CONSOLE

   The new process has a new console, instead of inheriting its
   parent's console (the default).

subprocess.CREATE_NEW_PROCESS_GROUP

   A "Popen" "creationflags" parameter to specify that a new process
   group will be created. This flag is necessary for using "os.kill()"
   on the subprocess.

   This flag is ignored if "CREATE_NEW_CONSOLE" is specified.


17.5.5. Older high-level API
============================

Prior to Python 3.5, these three functions comprised the high level
API to subprocess. You can now use "run()" in many cases, but lots of
existing code calls these functions.

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)

   Run the command described by *args*.  Wait for command to complete,
   then return the "returncode" attribute.

   This is equivalent to:

      run(...).returncode

   (except that the *input* and *check* parameters are not supported)

   The arguments shown above are merely the most common ones. The full
   function signature is largely the same as that of the "Popen"
   constructor - this function passes all supplied arguments other
   than *timeout* directly through to that interface.

   備註: Do not use "stdout=PIPE" or "stderr=PIPE" with this
     function. The child process will block if it generates enough
     output to a pipe to fill up the OS pipe buffer as the pipes are
     not being read from.

   3.3 版更變: *timeout* was added.

subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)

   Run command with arguments.  Wait for command to complete. If the
   return code was zero then return, otherwise raise
   "CalledProcessError". The "CalledProcessError" object will have the
   return code in the "returncode" attribute.

   This is equivalent to:

      run(..., check=True)

   (except that the *input* parameter is not supported)

   The arguments shown above are merely the most common ones. The full
   function signature is largely the same as that of the "Popen"
   constructor - this function passes all supplied arguments other
   than *timeout* directly through to that interface.

   備註: Do not use "stdout=PIPE" or "stderr=PIPE" with this
     function. The child process will block if it generates enough
     output to a pipe to fill up the OS pipe buffer as the pipes are
     not being read from.

   3.3 版更變: *timeout* was added.

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, cwd=None, encoding=None, errors=None, universal_newlines=False, timeout=None)

   Run command with arguments and return its output.

   If the return code was non-zero it raises a "CalledProcessError".
   The "CalledProcessError" object will have the return code in the
   "returncode" attribute and any output in the "output" attribute.

   This is equivalent to:

      run(..., check=True, stdout=PIPE).stdout

   The arguments shown above are merely the most common ones. The full
   function signature is largely the same as that of "run()" - most
   arguments are passed directly through to that interface. However,
   explicitly passing "input=None" to inherit the parent's standard
   input file handle is not supported.

   By default, this function will return the data as encoded bytes.
   The actual encoding of the output data may depend on the command
   being invoked, so the decoding to text will often need to be
   handled at the application level.

   This behaviour may be overridden by setting *universal_newlines* to
   "True" as described above in Frequently Used Arguments.

   To also capture standard error in the result, use
   "stderr=subprocess.STDOUT":

      >>> subprocess.check_output(
      ...     "ls non_existent_file; exit 0",
      ...     stderr=subprocess.STDOUT,
      ...     shell=True)
      'ls: non_existent_file: No such file or directory\n'

   3.1 版新加入.

   3.3 版更變: *timeout* was added.

   3.4 版更變: Support for the *input* keyword argument was added.

   3.6 版更變: *encoding* and *errors* were added.  See "run()" for
   details.


17.5.6. Replacing Older Functions with the "subprocess" Module
==============================================================

In this section, "a becomes b" means that b can be used as a
replacement for a.

備註: All "a" functions in this section fail (more or less) silently
  if the executed program cannot be found; the "b" replacements raise
  "OSError" instead.In addition, the replacements using
  "check_output()" will fail with a "CalledProcessError" if the
  requested operation produces a non-zero return code. The output is
  still available as the "output" attribute of the raised exception.

In the following examples, we assume that the relevant functions have
already been imported from the "subprocess" module.


17.5.6.1. Replacing /bin/sh shell backquote
-------------------------------------------

   output=`mycmd myarg`

becomes:

   output = check_output(["mycmd", "myarg"])


17.5.6.2. Replacing shell pipeline
----------------------------------

   output=`dmesg | grep hda`

becomes:

   p1 = Popen(["dmesg"], stdout=PIPE)
   p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
   p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
   output = p2.communicate()[0]

The p1.stdout.close() call after starting the p2 is important in order
for p1 to receive a SIGPIPE if p2 exits before p1.

Alternatively, for trusted input, the shell's own pipeline support may
still be used directly:

   output=`dmesg | grep hda`

becomes:

   output=check_output("dmesg | grep hda", shell=True)


17.5.6.3. Replacing "os.system()"
---------------------------------

   sts = os.system("mycmd" + " myarg")
   # becomes
   sts = call("mycmd" + " myarg", shell=True)

註解：

* Calling the program through the shell is usually not required.

A more realistic example would look like this:

   try:
       retcode = call("mycmd" + " myarg", shell=True)
       if retcode < 0:
           print("Child was terminated by signal", -retcode, file=sys.stderr)
       else:
           print("Child returned", retcode, file=sys.stderr)
   except OSError as e:
       print("Execution failed:", e, file=sys.stderr)


17.5.6.4. Replacing the "os.spawn" family
-----------------------------------------

P_NOWAIT example:

   pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
   ==>
   pid = Popen(["/bin/mycmd", "myarg"]).pid

P_WAIT example:

   retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
   ==>
   retcode = call(["/bin/mycmd", "myarg"])

Vector example:

   os.spawnvp(os.P_NOWAIT, path, args)
   ==>
   Popen([path] + args[1:])

Environment example:

   os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
   ==>
   Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})


17.5.6.5. Replacing "os.popen()", "os.popen2()", "os.popen3()"
--------------------------------------------------------------

   (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
   ==>
   p = Popen(cmd, shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, close_fds=True)
   (child_stdin, child_stdout) = (p.stdin, p.stdout)

   (child_stdin,
    child_stdout,
    child_stderr) = os.popen3(cmd, mode, bufsize)
   ==>
   p = Popen(cmd, shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
   (child_stdin,
    child_stdout,
    child_stderr) = (p.stdin, p.stdout, p.stderr)

   (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
   ==>
   p = Popen(cmd, shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
   (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)

Return code handling translates as follows:

   pipe = os.popen(cmd, 'w')
   ...
   rc = pipe.close()
   if rc is not None and rc >> 8:
       print("There were some errors")
   ==>
   process = Popen(cmd, stdin=PIPE)
   ...
   process.stdin.close()
   if process.wait() != 0:
       print("There were some errors")


17.5.6.6. Replacing functions from the "popen2" module
------------------------------------------------------

備註: If the cmd argument to popen2 functions is a string, the
  command is executed through /bin/sh.  If it is a list, the command
  is directly executed.

   (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
   ==>
   p = Popen("somestring", shell=True, bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, close_fds=True)
   (child_stdout, child_stdin) = (p.stdout, p.stdin)

   (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
   ==>
   p = Popen(["mycmd", "myarg"], bufsize=bufsize,
             stdin=PIPE, stdout=PIPE, close_fds=True)
   (child_stdout, child_stdin) = (p.stdout, p.stdin)

"popen2.Popen3" and "popen2.Popen4" basically work as
"subprocess.Popen", except that:

* "Popen" raises an exception if the execution fails.

* the *capturestderr* argument is replaced with the *stderr*
  argument.

* "stdin=PIPE" and "stdout=PIPE" must be specified.

* popen2 closes all file descriptors by default, but you have to
  specify "close_fds=True" with "Popen" to guarantee this behavior on
  all platforms or past Python versions.


17.5.7. Legacy Shell Invocation Functions
=========================================

This module also provides the following legacy functions from the 2.x
"commands" module. These operations implicitly invoke the system shell
and none of the guarantees described above regarding security and
exception handling consistency are valid for these functions.

subprocess.getstatusoutput(cmd)

   Return "(exitcode, output)" of executing *cmd* in a shell.

   Execute the string *cmd* in a shell with "Popen.check_output()" and
   return a 2-tuple "(exitcode, output)". The locale encoding is used;
   see the notes on Frequently Used Arguments for more details.

   A trailing newline is stripped from the output. The exit code for
   the command can be interpreted as the return code of subprocess.
   Example:

      >>> subprocess.getstatusoutput('ls /bin/ls')
      (0, '/bin/ls')
      >>> subprocess.getstatusoutput('cat /bin/junk')
      (1, 'cat: /bin/junk: No such file or directory')
      >>> subprocess.getstatusoutput('/bin/junk')
      (127, 'sh: /bin/junk: not found')
      >>> subprocess.getstatusoutput('/bin/kill $$')
      (-15, '')

   Availability: POSIX & Windows

   3.3.4 版更變: Windows support was added.The function now returns
   (exitcode, output) instead of (status, output) as it did in Python
   3.3.3 and earlier.  exitcode has the same value as "returncode".

subprocess.getoutput(cmd)

   Return output (stdout and stderr) of executing *cmd* in a shell.

   Like "getstatusoutput()", except the exit code is ignored and the
   return value is a string containing the command's output.  Example:

      >>> subprocess.getoutput('ls /bin/ls')
      '/bin/ls'

   Availability: POSIX & Windows

   3.3.4 版更變: Windows support added


17.5.8. 註解
============


17.5.8.1. Converting an argument sequence to a string on Windows
----------------------------------------------------------------

On Windows, an *args* sequence is converted to a string that can be
parsed using the following rules (which correspond to the rules used
by the MS C runtime):

1. Arguments are delimited by white space, which is either a space
   or a tab.

2. A string surrounded by double quotation marks is interpreted as
   a single argument, regardless of white space contained within.  A
   quoted string can be embedded in an argument.

3. A double quotation mark preceded by a backslash is interpreted
   as a literal double quotation mark.

4. Backslashes are interpreted literally, unless they immediately
   precede a double quotation mark.

5. If backslashes immediately precede a double quotation mark,
   every pair of backslashes is interpreted as a literal backslash.
   If the number of backslashes is odd, the last backslash escapes the
   next double quotation mark as described in rule 3.

也參考:

  "shlex"
     Module which provides function to parse and escape command lines.
