18.5.5.1. Create a subprocess

asyncio.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 Process instance.

This function is a coroutine.

asyncio.create_subprocess_exec(*args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)

Create a subprocess. Return a Process instance.

This function is a coroutine.

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

18.5.5.2. 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 stderr argument to create_subprocess_shell() and create_subprocess_exec() and indicates that standard error should go into the same handle as standard output.

18.5.5.3. Process

class asyncio.subprocess.Process

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.

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).

stdin

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

stdout

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

stderr

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

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 (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.

kill()

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

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.

wait(self):

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

18.5.5.4. Example

Implement a function similar to 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 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)

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()