.. currentmodule:: asyncio

.. _asyncio-subprocess:

============ Subprocesses

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

.. _asyncio_example_subprocess_shell:

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(*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.

.. important::

It is the application's responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid shell injection <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>_ vulnerabilities. The :func: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.

.. note::

The default asyncio event loop implementation on Windows does not support subprocesses. Subprocesses are available for Windows if a :class:ProactorEventLoop is used. See :ref:Subprocess Support on Windows <asyncio-windows-subprocess> for details.

.. 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.

.. class:: asyncio.subprocess.Process

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``.

.. warning::

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

.. 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).

.. _asyncio-subprocess-threads:

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.

.. _asyncio_example_create_subprocess_exec:

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.