cpython/Doc/library/asyncio-subprocess.rst

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.. currentmodule:: asyncio
Subprocess
==========
Operating system support
------------------------
On Windows, the default event loop uses :class:`selectors.SelectSelector`
which only supports sockets. The :class:`ProactorEventLoop` should be used to
support subprocesses.
On Mac OS X older than 10.9 (Mavericks), :class:`selectors.KqueueSelector`
does not support character devices like PTY, whereas it is used by the
default event loop. The :class:`SelectorEventLoop` can be used with
:class:`SelectSelector` or :class:`PollSelector` to handle character
devices on Mac OS X 10.6 (Snow Leopard) and later.
Create a subprocess: high-level API using Process
-------------------------------------------------
.. function:: create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, loop=None, limit=None, \*\*kwds)
Run the shell command *cmd* given as a string. Return a :class:`~asyncio.subprocess.Process`
instance.
The optional *limit* parameter sets the buffer limit passed to the
:class:`StreamReader`.
This function is a :ref:`coroutine <coroutine>`.
.. function:: create_subprocess_exec(\*args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, \*\*kwds)
Create a subprocess. Return a :class:`~asyncio.subprocess.Process` instance.
The optional *limit* parameter sets the buffer limit passed to the
:class:`StreamReader`.
This function is a :ref:`coroutine <coroutine>`.
Use the :meth:`BaseEventLoop.connect_read_pipe` and
:meth:`BaseEventLoop.connect_write_pipe` methods to connect pipes.
Create a subprocess: low-level API using subprocess.Popen
---------------------------------------------------------
Run subprocesses asynchronously using the :mod:`subprocess` module.
.. method:: BaseEventLoop.subprocess_exec(protocol_factory, \*args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, \*\*kwargs)
Create a subprocess from one or more string arguments, 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.
Other parameters:
* *stdin*: Either a file-like object representing the pipe to be connected
to the subprocess's standard input stream using
:meth:`~BaseEventLoop.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:`~BaseEventLoop.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:`~BaseEventLoop.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.
.. method:: BaseEventLoop.subprocess_shell(protocol_factory, cmd, \*, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, \*\*kwargs)
Create a subprocess from *cmd*, which is a string using the platform's
"shell" syntax. This is similar to the standard library
:class:`subprocess.Popen` class called with ``shell=True``.
See :meth:`~BaseEventLoop.subprocess_exec` for more details about
the remaining arguments.
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.
.. seealso::
The :meth:`BaseEventLoop.connect_read_pipe` and
:meth:`BaseEventLoop.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
.. attribute:: pid
The identifier of the process.
Note that if you set the *shell* argument to ``True``, this 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).
.. attribute:: stdin
Standard input stream (write), ``None`` if the process was created with
``stdin=None``.
.. attribute:: stdout
Standard output stream (read), ``None`` if the process was created with
``stdout=None``.
.. attribute:: stderr
Standard error stream (read), ``None`` if the process was created with
``stderr=None``.
.. method:: 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 ``(stdoutdata, stderrdata)``.
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.
.. note::
The data read is buffered in memory, so do not use this method if the
data size is large or unlimited.
This method is a :ref:`coroutine <coroutine>`.
.. 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`.
.. 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:: wait():
Wait for child process to terminate. Set and return :attr:`returncode`
attribute.
This method is a :ref:`coroutine <coroutine>`.
Example
-------
Implement a function similar to :func:`subprocess.getstatusoutput`, except that
it does not use a shell. Get the output of the "python -m platform" command and
display the output::
import asyncio
import os
import sys
from asyncio import subprocess
@asyncio.coroutine
def getstatusoutput(*args):
proc = yield from asyncio.create_subprocess_exec(
*args,
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT)
try:
stdout, _ = yield from proc.communicate()
except:
proc.kill()
yield from proc.wait()
raise
exitcode = yield from proc.wait()
return (exitcode, stdout)
if os.name == 'nt':
loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)
else:
loop = asyncio.get_event_loop()
coro = getstatusoutput(sys.executable, '-m', 'platform')
exitcode, stdout = loop.run_until_complete(coro)
if not exitcode:
stdout = stdout.decode('ascii').rstrip()
print("Platform: %s" % stdout)
else:
print("Python failed with exit code %s:" % exitcode)
sys.stdout.flush()
sys.stdout.buffer.flush()
sys.stdout.buffer.write(stdout)
sys.stdout.buffer.flush()
loop.close()