How to stream stdout/stderr from a child process using asyncio, and obtain its exit code after?

Question

Under Python 3.4 on Windows, I need to stream data written to stdout/stderr by a child process, i.e. receive its output as it occurs, using the asyncio framework introduced in Python 3.4. I also have to determine the program's exit code afterwards. How can I do this?

Answer 1

The solution I've come up with so far uses SubprocessProtocol to receive output from the child process, and the associated transport to get the process' exit code. I don't know if this is optimal though. I've based my approach on an answer to a similar question by J.F. Sebastian.

import asyncio
import contextlib
import os
import locale


class SubprocessProtocol(asyncio.SubprocessProtocol):
    def pipe_data_received(self, fd, data):
        if fd == 1:
            name = 'stdout'
        elif fd == 2:
            name = 'stderr'
        text = data.decode(locale.getpreferredencoding(False))
        print('Received from {}: {}'.format(name, text.strip()))

    def process_exited(self):
        loop.stop()


if os.name == 'nt':
    # On Windows, the ProactorEventLoop is necessary to listen on pipes
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)
else:
    loop = asyncio.get_event_loop()
with contextlib.closing(loop):
    # This will only connect to the process
    transport = loop.run_until_complete(loop.subprocess_exec(
        SubprocessProtocol, 'python', '-c', 'print(\'Hello async world!\')'))[0]
    # Wait until process has finished
    loop.run_forever()
    print('Program exited with: {}'.format(transport.get_returncode()))