.. currentmodule:: asyncio
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>`.
.. 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.
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 instanciate 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 instanciate 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.
.. 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.
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 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.
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)
@asyncio.coroutine
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
create = loop.subprocess_exec(lambda: DateProtocol(exit_future),
sys.executable, '-c', code,
stdin=None, stderr=None)
transport, protocol = yield from create
# Wait for the subprocess exit using the process_exited() method
# of the protocol
yield from 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()
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
create = asyncio.create_subprocess_exec(sys.executable, '-c', code,
stdout=asyncio.subprocess.PIPE)
proc = yield from create
# Read one line of output
data = yield from proc.stdout.readline()
line = data.decode('ascii').rstrip()
# Wait for the subprocess exit
yield from 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()