Python 3.4+Async programmingterm

asyncio.SubprocessProtocol

protocols and transports

The protocol for a child process: the callbacks pipe_data_received(fd, data) for stdout/stderr output, pipe_connection_lost() and process_exited().

Syntax

transport, protocol = await loop.subprocess_exec(ProtocolClass, program, *args)

Learners rarely need this layer: streams (asyncio.open_connection() and asyncio.start_server()) cover TCP, and asyncio.create_subprocess_exec() covers processes. Instead of a protocol, asyncio.create_subprocess_exec() with await proc.communicate() is almost always easier: the output arrives whole, with no sorting of which fd sent what. The protocol's trap: process_exited() may come before the last pipe output is read, so 'the process exited' does not mean 'all data has arrived'.

Examples

import asyncio
import sys
class Out(asyncio.SubprocessProtocol):
    def __init__(self, done):
        self.done = done
        self.data = b''
    def pipe_data_received(self, fd, data):
        self.data += data
    def connection_lost(self, exc):
        self.done.set_result(True)  # все каналы закрыты — вывод получен целиком
async def main():
    loop = asyncio.get_running_loop()
    done = loop.create_future()
    transport, proto = await loop.subprocess_exec(
        lambda: Out(done), sys.executable, '-c', 'print(6 * 7)', stdin=None)
    await done
    print(proto.data.strip(), transport.get_returncode())  # → b'42' 0
    transport.close()
asyncio.run(main())

See also

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