asyncio-subprocess.rst

.. currentmodule:: asyncio

.. _asyncio-subprocess:

Subprocess
==========

**Source code:** :source:`Lib/asyncio/subprocess.py`

Windows event loop
------------------

On Windows, the default event loop is :class:`SelectorEventLoop` which does not
support subprocesses. :class:`ProactorEventLoop` should be used instead.
Example to use it on Windows::

    import asyncio, sys

    if sys.platform == 'win32':
        loop = asyncio.ProactorEventLoop()
        asyncio.set_event_loop(loop)

.. seealso::

   :ref:`Available event loops <asyncio-event-loops>` and :ref:`Platform
   support <asyncio-platform-support>`.


Create a subprocess: high-level API using Process
-------------------------------------------------

.. coroutinefunction:: create_subprocess_exec(\*args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, \*\*kwds)

   Create a subprocess.

   The *limit* parameter sets the buffer limit passed to the
   :class:`StreamReader`. See :meth:`AbstractEventLoop.subprocess_exec` for other
   parameters.

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

   This function is a :ref:`coroutine <coroutine>`.

.. coroutinefunction:: create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, loop=None, limit=None, \*\*kwds)

   Run the shell command *cmd*.

   The *limit* parameter sets the buffer limit passed to the
   :class:`StreamReader`. See :meth:`AbstractEventLoop.subprocess_shell` for other
   parameters.

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

   It is the application's responsibility to ensure that all whitespace and
   metacharacters 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 shell metacharacters in strings that are going to be
   used to construct shell commands.

   This function is a :ref:`coroutine <coroutine>`.

Use the :meth:`AbstractEventLoop.connect_read_pipe` and
:meth:`AbstractEventLoop.connect_write_pipe` methods to connect pipes.


Create a subprocess: low-level API using subprocess.Popen
---------------------------------------------------------

Run subprocesses asynchronously using the :mod:`subprocess` module.

.. coroutinemethod:: AbstractEventLoop.subprocess_exec(protocol_factory, \*args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, \*\*kwargs)

   Create a subprocess from one or more string arguments (character strings or
   bytes strings encoded to the :ref:`filesystem encoding
   <filesystem-encoding>`), where the first string
   specifies the program to execute, and the remaining strings specify the
   program's arguments. (Thus, together the string arguments form the
   ``sys.argv`` value of the program, assuming it is a Python script.) This is
   similar to the standard library :class:`subprocess.Popen` class called with
   shell=False and the list of strings passed as the first argument;
   however, where :class:`~subprocess.Popen` takes a single argument which is
   list of strings, :func:`subprocess_exec` takes multiple string arguments.

   The *protocol_factory* must instantiate a subclass of the
   :class:`asyncio.SubprocessProtocol` class.

   Other parameters:

   * *stdin*: Either a file-like object representing the pipe to be connected
     to the subprocess's standard input stream using
     :meth:`~AbstractEventLoop.connect_write_pipe`, or the constant
     :const:`subprocess.PIPE` (the default). By default a new pipe will be
     created and connected.

   * *stdout*: Either a file-like object representing the pipe to be connected
     to the subprocess's standard output stream using
     :meth:`~AbstractEventLoop.connect_read_pipe`, or the constant
     :const:`subprocess.PIPE` (the default). By default a new pipe will be
     created and connected.

   * *stderr*: Either a file-like object representing the pipe to be connected
     to the subprocess's standard error stream using
     :meth:`~AbstractEventLoop.connect_read_pipe`, or one of the constants
     :const:`subprocess.PIPE` (the default) or :const:`subprocess.STDOUT`.
     By default a new pipe will be created and connected. When
     :const:`subprocess.STDOUT` is specified, the subprocess's standard error
     stream will be connected to the same pipe as the standard output stream.

   * All other keyword arguments are passed to :class:`subprocess.Popen`
     without interpretation, except for *bufsize*, *universal_newlines* and
     *shell*, which should not be specified at all.

   Returns a pair of ``(transport, protocol)``, where *transport* is an
   instance of :class:`BaseSubprocessTransport`.

   This method is a :ref:`coroutine <coroutine>`.

   See the constructor of the :class:`subprocess.Popen` class for parameters.

.. coroutinemethod:: AbstractEventLoop.subprocess_shell(protocol_factory, cmd, \*, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, \*\*kwargs)

   Create a subprocess from *cmd*, which is a character string or a bytes
   string encoded to the :ref:`filesystem encoding <filesystem-encoding>`,
   using the platform's "shell" syntax. This is similar to the standard library
   :class:`subprocess.Popen` class called with ``shell=True``.

   The *protocol_factory* must instantiate a subclass of the
   :class:`asyncio.SubprocessProtocol` class.

   See :meth:`~AbstractEventLoop.subprocess_exec` for more details about
   the remaining arguments.

   Returns a pair of ``(transport, protocol)``, where *transport* is an
   instance of :class:`BaseSubprocessTransport`.

   It is the application's responsibility to ensure that all whitespace and
   metacharacters 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 shell metacharacters in strings that are going to be
   used to construct shell commands.

   This method is a :ref:`coroutine <coroutine>`.

.. seealso::

   The :meth:`AbstractEventLoop.connect_read_pipe` and
   :meth:`AbstractEventLoop.connect_write_pipe` methods.


Constants
---------

.. data:: asyncio.subprocess.PIPE

   Special value that can be used as the *stdin*, *stdout* or *stderr* argument
   to :func:`create_subprocess_shell` and :func:`create_subprocess_exec` and
   indicates that a pipe to the standard stream should be opened.

.. data:: asyncio.subprocess.STDOUT

   Special value that can be used as the *stderr* argument to
   :func:`create_subprocess_shell` and :func:`create_subprocess_exec` and
   indicates that standard error should go into the same handle as standard
   output.

.. data:: asyncio.subprocess.DEVNULL

   Special value that can be used as the *stdin*, *stdout* or *stderr* argument
   to :func:`create_subprocess_shell` and :func:`create_subprocess_exec` and
   indicates that the special file :data:`os.devnull` will be used.


Process
-------

.. class:: asyncio.subprocess.Process

   A subprocess created by the :func:`create_subprocess_exec` or the
   :func:`create_subprocess_shell` function.

   The API of the :class:`~asyncio.subprocess.Process` class was designed to be
   close to the API of the :class:`subprocess.Popen` class, but there are some
   differences:

   * There is no explicit :meth:`~subprocess.Popen.poll` method
   * The :meth:`~subprocess.Popen.communicate` and
     :meth:`~subprocess.Popen.wait` methods don't take a *timeout* parameter:
     use the :func:`wait_for` function
   * The *universal_newlines* parameter is not supported (only bytes strings
     are supported)
   * The :meth:`~asyncio.subprocess.Process.wait` method of
     the :class:`~asyncio.subprocess.Process` class is asynchronous whereas the
     :meth:`~subprocess.Popen.wait` method of the :class:`~subprocess.Popen`
     class is implemented as a busy loop.

   This class is :ref:`not thread safe <asyncio-multithreading>`. See also the
   :ref:`Subprocess and threads <asyncio-subprocess-threads>` section.

   .. coroutinemethod:: wait()

      Wait for child process to terminate.  Set and return :attr:`returncode`
      attribute.

      This method is a :ref:`coroutine <coroutine>`.

      .. note::

         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 the
         :meth:`communicate` method when using pipes to avoid that.

   .. coroutinemethod:: communicate(input=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.  The type
      of *input* must be bytes.

      :meth:`communicate` returns a tuple ``(stdout_data, stderr_data)``.

      If a :exc:`BrokenPipeError` or :exc:`ConnectionResetError` exception is
      raised when writing *input* into stdin, the exception is ignored. It
      occurs when the process exits before all data are written into stdin.

      Note that if you want to send data to the process's stdin, you need to
      create the Process 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.

      This method is a :ref:`coroutine <coroutine>`.

      .. note::

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

      .. versionchanged:: 3.4.2
         The method now ignores :exc:`BrokenPipeError` and
         :exc:`ConnectionResetError`.

   .. 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. On Posix OSs the method sends :py:data:`signal.SIGTERM`
      to the child. On Windows the Win32 API function
      :c:func:`TerminateProcess` is called to stop the child.

   .. method:: kill()

      Kills the child. On Posix OSs the function sends :py:data:`SIGKILL` to
      the child.  On Windows :meth:`kill` is an alias for :meth:`terminate`.

   .. attribute:: stdin

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

   .. attribute:: stdout

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

   .. attribute:: stderr

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

   .. warning::

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

   .. attribute:: pid

      The identifier of the process.

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

   .. attribute:: returncode

      Return code of the process when it exited.  A ``None`` value indicates
      that the process has not terminated yet.

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


.. _asyncio-subprocess-threads:

Subprocess and threads
----------------------

asyncio supports running subprocesses from different threads, but there
are limits:

* 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.

The :class:`asyncio.subprocess.Process` class is not thread safe.

.. seealso::

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


Subprocess examples
-------------------

Subprocess using transport and protocol
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example of a subprocess protocol using to get the output of a subprocess and to
wait for the subprocess exit. The subprocess is created by the
:meth:`AbstractEventLoop.subprocess_exec` method::

    import asyncio
    import sys

    class DateProtocol(asyncio.SubprocessProtocol):
        def __init__(self, exit_future):
            self.exit_future = exit_future
            self.output = bytearray()

        def pipe_data_received(self, fd, data):
            self.output.extend(data)

        def process_exited(self):
            self.exit_future.set_result(True)

    async def get_date(loop):
        code = 'import datetime; print(datetime.datetime.now())'
        exit_future = asyncio.Future(loop=loop)

        # Create the subprocess controlled by the protocol DateProtocol,
        # redirect the standard output into a pipe
        transport, protocol = await loop.subprocess_exec(
            lambda: DateProtocol(exit_future),
            sys.executable, '-c', code,
            stdin=None, stderr=None)

        # Wait for the subprocess exit using the process_exited() method
        # of the protocol
        await exit_future

        # Close the stdout pipe
        transport.close()

        # Read the output which was collected by the pipe_data_received()
        # method of the protocol
        data = bytes(protocol.output)
        return data.decode('ascii').rstrip()

    if sys.platform == "win32":
        loop = asyncio.ProactorEventLoop()
        asyncio.set_event_loop(loop)
    else:
        loop = asyncio.get_event_loop()

    date = loop.run_until_complete(get_date(loop))
    print("Current date: %s" % date)
    loop.close()


Subprocess using streams
^^^^^^^^^^^^^^^^^^^^^^^^

Example using the :class:`~asyncio.subprocess.Process` class to control the
subprocess and the :class:`StreamReader` class to read from the standard
output.  The subprocess is created by the :func:`create_subprocess_exec`
function::

    import asyncio.subprocess
    import sys

    @asyncio.coroutine
    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":
        loop = asyncio.ProactorEventLoop()
        asyncio.set_event_loop(loop)
    else:
        loop = asyncio.get_event_loop()

    date = loop.run_until_complete(get_date())
    print("Current date: %s" % date)
    loop.close()