.. currentmodule:: asyncio

Subprocesses

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

.. coroutinefunction:: 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 :class:`StreamReader`
 wrappers for :attr:`Process.stdout` and :attr:`Process.stderr`
 (if :attr:`subprocess.PIPE` is passed to *stdout* and *stderr* arguments).

 Return a :class:`~asyncio.subprocess.Process` instance.

 See the documentation of :meth:`loop.subprocess_exec` for other
 parameters.

.. coroutinefunction:: 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 :class:`StreamReader`
 wrappers for :attr:`Process.stdout` and :attr:`Process.stderr`
 (if :attr:`subprocess.PIPE` is passed to *stdout* and *stderr* arguments).

 Return a :class:`~asyncio.subprocess.Process` instance.

 See the documentation of :meth:`loop.subprocess_shell` for other
 parameters.

.. seealso::

 asyncio also has the following *low-level* APIs to work with subprocesses:
 :meth:`loop.subprocess_exec`, :meth:`loop.subprocess_shell`,
 :meth:`loop.connect_read_pipe`, :meth:`loop.connect_write_pipe`,
 as well as the :ref:`Subprocess Transports <asyncio-subprocess-transports>`
 and :ref:`Subprocess Protocols <asyncio-subprocess-protocols>`.

Constants

.. data:: asyncio.subprocess.PIPE

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

 If *PIPE* is passed to *stdin* argument, the
 :attr:`Process.stdin <asyncio.subprocess.Process.stdin>` attribute
 will point to a :class:`StreamWriter` instance.

 If *PIPE* is passed to *stdout* or *stderr* arguments, the
 :attr:`Process.stdout <asyncio.subprocess.Process.stdout>` and
 :attr:`Process.stderr <asyncio.subprocess.Process.stderr>`
 attributes will point to :class:`StreamReader` instances.

.. data:: asyncio.subprocess.STDOUT

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

.. data:: 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
 :data:`os.devnull` will be used for the corresponding subprocess stream.

Interacting with Subprocesses

Both :func:`create_subprocess_exec` and :func:`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.

An object that wraps OS processes created by the :func:`create_subprocess_exec` and :func:`create_subprocess_shell` functions.

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

• unlike Popen, Process instances do not have an equivalent to the :meth:`~subprocess.Popen.poll` method;
• the :meth:`~asyncio.subprocess.Process.communicate` and :meth:`~asyncio.subprocess.Process.wait` methods don't have a timeout parameter: use the :func:`wait_for` function;
• the :meth:`Process.wait() <asyncio.subprocess.Process.wait>` method is asynchronous, whereas :meth:`subprocess.Popen.wait` method is implemented as a blocking busy loop;
• the universal_newlines parameter is not supported.

This class is :ref:`not thread safe <asyncio-multithreading>`.

See also the :ref:`Subprocess and Threads <asyncio-subprocess-threads>` section.

.. coroutinemethod:: wait()

 Wait for the child process to terminate.

 Set and return the :attr:`returncode` attribute.

 .. note::

 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 :meth:`communicate` method when using pipes
 to avoid this condition.

.. coroutinemethod:: 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 (:class:`bytes` object)
 that will be sent to the child process.

 Return a tuple ``(stdout_data, stderr_data)``.

 If either :exc:`BrokenPipeError` or :exc:`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.

.. method:: send_signal(signal)

 Sends the signal *signal* to the child process.

 .. note::

 On Windows, :py:data:`SIGTERM` is an alias for :meth:`terminate`.
 ``CTRL_C_EVENT`` and ``CTRL_BREAK_EVENT`` can be sent to processes
 started with a *creationflags* parameter which includes
 ``CREATE_NEW_PROCESS_GROUP``.

.. method:: terminate()

 Stop the child process.

 On POSIX systems this method sends :py:data:`signal.SIGTERM` to the
 child process.

 On Windows the Win32 API function :c:func:`TerminateProcess` is
 called to stop the child process.

.. method:: kill()

 Kill the child.

 On POSIX systems this method sends :py:data:`SIGKILL` to the child
 process.

 On Windows this method is an alias for :meth:`terminate`.

.. attribute:: stdin

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

.. attribute:: stdout

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

.. attribute:: stderr

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

.. attribute:: pid

 Process identification number (PID).

 Note that for processes created by the :func:`create_subprocess_shell`
 function, this attribute is the PID of the spawned shell.

.. attribute:: 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, but there are limitations:

• An event loop must run in the main thread.
• The child watcher must be instantiated in the main thread before executing subprocesses from other threads. Call the :func:`get_child_watcher` function in the main thread to instantiate the child watcher.

Note that alternative event loop implementations might not share the above limitations; please refer to their documentation.

.. seealso::

 The :ref:`Concurrency and multithreading in asyncio
 <asyncio-multithreading>` section.

Examples

An example using the :class:`~asyncio.subprocess.Process` class to control a subprocess and the :class:`StreamReader` class to read from its standard output.

The subprocess is created by the :func:`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

if sys.platform == "win32":
 asyncio.set_event_loop_policy(
 asyncio.WindowsProactorEventLoopPolicy())

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

See also the :ref:`same example <asyncio_example_subprocess_proto>` written using low-level APIs.

此处可能存在不合适展示的内容,页面不予展示。您可通过相关编辑功能自查并修改。

如您确认内容无涉及 不当用语 / 纯广告导流 / 暴力 / 低俗色情 / 侵权 / 盗版 / 虚假 / 无价值内容或违法国家有关法律法规的内容,可点击提交进行申诉,我们将尽快为您处理。