Subprocess

1. Windows event loop

On Windows, the default event loop is SelectorEventLoop which does not support subprocesses. 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)

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

coroutine asyncio.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 StreamReader. See AbstractEventLoop.subprocess_exec() for other parameters.

Return a Process instance.

This function is a coroutine.

coroutine asyncio.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 StreamReader. See AbstractEventLoop.subprocess_shell() for other parameters.

Return a Process instance.

It is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities. The 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 coroutine.

Use the AbstractEventLoop.connect_read_pipe() and AbstractEventLoop.connect_write_pipe() methods to connect pipes.

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

Run subprocesses asynchronously using the subprocess module.

coroutine 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 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 subprocess.Popen class called with shell=False and the list of strings passed as the first argument; however, where Popen takes a single argument which is list of strings, subprocess_exec() takes multiple string arguments.

The protocol_factory must instantiate a subclass of the 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 connect_write_pipe(), or the constant 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 connect_read_pipe(), or the constant 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 connect_read_pipe(), or one of the constants subprocess.PIPE (the default) or subprocess.STDOUT. By default a new pipe will be created and connected. When 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 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 BaseSubprocessTransport.

This method is a coroutine.

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

coroutine 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 filesystem encoding, using the platform’s “shell” syntax. This is similar to the standard library subprocess.Popen class called with shell=True.

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

See subprocess_exec() for more details about the remaining arguments.

Returns a pair of (transport, protocol), where transport is an instance of BaseSubprocessTransport.

It is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities. The 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 coroutine.

4. Constants

asyncio.subprocess.PIPE

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

asyncio.subprocess.STDOUT

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

asyncio.subprocess.DEVNULL

Special value that can be used as the stdin, stdout or stderr argument to create_subprocess_shell() and create_subprocess_exec() and indicates that the special file os.devnull will be used.

5. Process

class asyncio.subprocess.Process

A subprocess created by the create_subprocess_exec() or the create_subprocess_shell() function.

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

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

This class is not thread safe. See also the Subprocess and threads section.

coroutine wait()

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

This method is a 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 communicate() method when using pipes to avoid that.

coroutine 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.

communicate() returns a tuple (stdout_data, stderr_data).

If a BrokenPipeError or 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 coroutine.

Note

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

Changed in version 3.4.2: The method now ignores BrokenPipeError and ConnectionResetError.

send_signal(signal)

Sends the signal signal to the child process.

Note

On Windows, SIGTERM is an alias for terminate(). CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP.

terminate()

Stop the child. On Posix OSs the method sends signal.SIGTERM to the child. On Windows the Win32 API function TerminateProcess() is called to stop the child.

kill()

Kills the child. On Posix OSs the function sends SIGKILL to the child. On Windows kill() is an alias for terminate().

stdin

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

stdout

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

stderr

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

Warning

Use the communicate() method rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to streams pausing reading or writing and blocking the child process.

pid

The identifier of the process.

Note that for processes created by the create_subprocess_shell() function, this attribute is the process identifier of the spawned shell.

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).

6. 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 get_child_watcher() function in the main thread to instantiate the child watcher.

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

7. Subprocess examples

7.1. 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 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()

7.2. Subprocess using streams

Example using the Process class to control the subprocess and the StreamReader class to read from the standard output. The subprocess is created by the 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()