子进程
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**Source code:** Lib/asyncio/subprocess.py,
Lib/asyncio/base_subprocess.py

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

This section describes high-level async/await asyncio APIs to create
and manage subprocesses.

Here's an example of how asyncio can run a shell command and obtain
its result:

   import asyncio

   async def run(cmd):
       proc = await asyncio.create_subprocess_shell(
           cmd,
           stdout=asyncio.subprocess.PIPE,
           stderr=asyncio.subprocess.PIPE)

       stdout, stderr = await proc.communicate()

       print(f'[{cmd!r} exited with {proc.returncode}]')
       if stdout:
           print(f'[stdout]\n{stdout.decode()}')
       if stderr:
           print(f'[stderr]\n{stderr.decode()}')

   asyncio.run(run('ls /zzz'))

will print:

   ['ls /zzz' exited with 1]
   [stderr]
   ls: /zzz: No such file or directory

Because all asyncio subprocess functions are asynchronous and asyncio
provides many tools to work with such functions, it is easy to execute
and monitor multiple subprocesses in parallel.  It is indeed trivial
to modify the above example to run several commands simultaneously:

   async def main():
       await asyncio.gather(
           run('ls /zzz'),
           run('sleep 1; echo "hello"'))

   asyncio.run(main())

See also the Examples subsection.


Creating Subprocesses
=====================

coroutine asyncio.create_subprocess_exec(program, *args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)

   Create a subprocess.

   The *limit* argument sets the buffer limit for "StreamReader"
   wrappers for "Process.stdout" and "Process.stderr" (if
   "subprocess.PIPE" is passed to *stdout* and *stderr* arguments).

   Return a "Process" instance.

   See the documentation of "loop.subprocess_exec()" for other
   parameters.

   Deprecated since version 3.8, will be removed in version 3.10:
   *loop* 形参。

coroutine asyncio.create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)

   Run the *cmd* shell command.

   The *limit* argument sets the buffer limit for "StreamReader"
   wrappers for "Process.stdout" and "Process.stderr" (if
   "subprocess.PIPE" is passed to *stdout* and *stderr* arguments).

   Return a "Process" instance.

   See the documentation of "loop.subprocess_shell()" for other
   parameters.

重要: It is the application's responsibility to ensure that all
  whitespace and special characters are quoted appropriately to avoid
  shell injection vulnerabilities. The "shlex.quote()" function can be
  used to properly escape whitespace and special shell characters in
  strings that are going to be used to construct shell commands.

  Deprecated since version 3.8, will be removed in version 3.10:
  *loop* 形参。

備註: The default asyncio event loop implementation on **Windows**
  does not support subprocesses. Subprocesses are available for
  Windows if a "ProactorEventLoop" is used. See Subprocess Support on
  Windows for details.

也參考: asyncio also has the following *low-level* APIs to work with
  subprocesses: "loop.subprocess_exec()", "loop.subprocess_shell()",
  "loop.connect_read_pipe()", "loop.connect_write_pipe()", as well as
  the Subprocess Transports and Subprocess Protocols.


Constants
=========

asyncio.subprocess.PIPE

   Can be passed to the *stdin*, *stdout* or *stderr* parameters.

   If *PIPE* is passed to *stdin* argument, the "Process.stdin"
   attribute will point to a "StreamWriter" instance.

   If *PIPE* is passed to *stdout* or *stderr* arguments, the
   "Process.stdout" and "Process.stderr" attributes will point to
   "StreamReader" instances.

asyncio.subprocess.STDOUT

   Special value that can be used as the *stderr* argument and
   indicates that standard error should be redirected into standard
   output.

asyncio.subprocess.DEVNULL

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to process creation functions.  It indicates that the
   special file "os.devnull" will be used for the corresponding
   subprocess stream.


Interacting with Subprocesses
=============================

Both "create_subprocess_exec()" and "create_subprocess_shell()"
functions return instances of the *Process* class.  *Process* is a
high-level wrapper that allows communicating with subprocesses and
watching for their completion.

class asyncio.subprocess.Process

   An object that wraps OS processes created by the
   "create_subprocess_exec()" and "create_subprocess_shell()"
   functions.

   This class is designed to have a similar API to the
   "subprocess.Popen" class, but there are some notable differences:

   * unlike Popen, Process instances do not have an equivalent to
     the "poll()" method;

   * the "communicate()" and "wait()" methods don't have a *timeout*
     parameter: use the "wait_for()" function;

   * the "Process.wait()" method is asynchronous, whereas
     "subprocess.Popen.wait()" method is implemented as a blocking
     busy loop;

   * the *universal_newlines* parameter is not supported.

   This class is not thread safe.

   See also the Subprocess and Threads section.

   coroutine wait()

      Wait for the child process to terminate.

      Set and return the "returncode" attribute.

      備註: This method can deadlock when using "stdout=PIPE" or
        "stderr=PIPE" and the child process generates so much output
        that it blocks waiting for the OS pipe buffer to accept more
        data. Use the "communicate()" method when using pipes to avoid
        this condition.

   coroutine communicate(input=None)

      Interact with process:

      1. send data to *stdin* (if *input* is not "None");

      2. read data from *stdout* and *stderr*, until EOF is
         reached;

      3. wait for process to terminate.

      The optional *input* argument is the data ("bytes" object) that
      will be sent to the child process.

      Return a tuple "(stdout_data, stderr_data)".

      If either "BrokenPipeError" or "ConnectionResetError" exception
      is raised when writing *input* into *stdin*, the exception is
      ignored.  This condition occurs when the process exits before
      all data are written into *stdin*.

      If it is desired to send data to the process' *stdin*, the
      process needs to be created with "stdin=PIPE".  Similarly, to
      get anything other than "None" in the result tuple, the process
      has to be created with "stdout=PIPE" and/or "stderr=PIPE"
      arguments.

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

   send_signal(signal)

      Sends the signal *signal* to the child process.

      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".

   terminate()

      Stop the child process.

      On POSIX systems this method sends "signal.SIGTERM" to the child
      process.

      On Windows the Win32 API function "TerminateProcess()" is called
      to stop the child process.

   kill()

      Kill the child.

      On POSIX systems this method sends "SIGKILL" to the child
      process.

      On Windows this method is an alias for "terminate()".

   stdin

      Standard input stream ("StreamWriter") or "None" if the process
      was created with "stdin=None".

   stdout

      Standard output stream ("StreamReader") or "None" if the process
      was created with "stdout=None".

   stderr

      Standard error stream ("StreamReader") or "None" if the process
      was created with "stderr=None".

   警告: Use the "communicate()" method rather than
     "process.stdin.write()", "await process.stdout.read()" or "await
     process.stderr.read". This avoids deadlocks due to streams
     pausing reading or writing and blocking the child process.

   pid

      Process identification number (PID).

      Note that for processes created by the
      "create_subprocess_shell()" function, this attribute is the PID
      of the spawned shell.

   returncode

      Return code of the process when it exits.

      A "None" value indicates that the process has not terminated
      yet.

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


Subprocess and Threads
----------------------

Standard asyncio event loop supports running subprocesses from
different threads by default.

On Windows subprocesses are provided by "ProactorEventLoop" only
(default), "SelectorEventLoop" has no subprocess support.

On UNIX *child watchers* are used for subprocess finish waiting, see
Process Watchers for more info.

3.8 版更變: UNIX switched to use "ThreadedChildWatcher" for spawning
subprocesses from different threads without any limitation.Spawning a
subprocess with *inactive* current child watcher raises
"RuntimeError".

Note that alternative event loop implementations might have own
limitations; please refer to their documentation.

也參考: The Concurrency and multithreading in asyncio section.


Examples
--------

An example using the "Process" class to control a subprocess and the
"StreamReader" class to read from its standard output.

The subprocess is created by the "create_subprocess_exec()" function:

   import asyncio
   import sys

   async def get_date():
       code = 'import datetime; print(datetime.datetime.now())'

       # Create the subprocess; redirect the standard output
       # into a pipe.
       proc = await asyncio.create_subprocess_exec(
           sys.executable, '-c', code,
           stdout=asyncio.subprocess.PIPE)

       # Read one line of output.
       data = await proc.stdout.readline()
       line = data.decode('ascii').rstrip()

       # Wait for the subprocess exit.
       await proc.wait()
       return line

   date = asyncio.run(get_date())
   print(f"Current date: {date}")

See also the same example written using low-level APIs.
