Simultaneously read stdout and stderr from Python `asyncio` create_subprocess_exec

asyncio-subprocess.py

import asyncio

async def run():
	proc = await asyncio.create_subprocess_exec(
		'/bin/ls', '/etc', '/not-exists',
		stdout=asyncio.subprocess.PIPE,
		stderr=asyncio.subprocess.PIPE
	)

	# Prepare set of asynchronous readline() tasks for `stdout` and `stderr` streams
	aws = {
		asyncio.create_task(proc.stdout.readline(), name="stdout"),
		asyncio.create_task(proc.stderr.readline(), name="stderr"),
	}

	while len(aws) > 0:

		done, aws = await asyncio.wait(aws, return_when=asyncio.FIRST_COMPLETED)
		for task in done:
			line = await task
			
			# If the line is empty, we are at the end of the stream
			if len(line) == 0:
				continue

			print(">>", task.get_name(), line)

			# Re-create a readline() task for a respective stream
			if task.get_name() == "stdout":
				aws.add(asyncio.create_task(proc.stdout.readline(), name="stdout"))
			elif task.get_name() == "stderr":
				aws.add(asyncio.create_task(proc.stderr.readline(), name="stderr"))

	# Wait till the process is finished
	returncode = await proc.wait()

	print('Exited with code', returncode)

asyncio.run(run())