How to create asyncio stream reader/writer for stdin/stdout?

I need to write two prorgams which will be run as a parent process and its child. The parent process spawns the child and then they communicate via pair of pipes connected to child's stdin and stdout. The communication is peer-to-peer, that's why I need asyncio. A simple read/reply loop won't do.

I have written the parent. No problem because asyncio provides everything I needed in create_subprocess_exec().

However I don't know how to create a similar stream reader/writer in the child. I did not expect any problems. because the pipes are already created and file descriptors 0 and 1 are ready to use when the child process starts. No connection is to be open, no process needs to be spawned.

This is my not working attempt:

import asyncio
import sys

_DEFAULT_LIMIT = 64 * 1024

async def connect_stdin_stdout(limit=_DEFAULT_LIMIT, loop=None):
    if loop is None:
        loop = asyncio.get_event_loop()
    reader = asyncio.StreamReader(limit=limit, loop=loop)
    protocol = asyncio.StreamReaderProtocol(reader, loop=loop)
    r_transport, _ = await loop.connect_read_pipe(lambda: protocol, sys.stdin)
    w_transport, _ = await loop.connect_write_pipe(lambda: protocol, sys.stdout)
    writer = asyncio.StreamWriter(w_transport, protocol, reader, loop)
    return reader, writer

The problem is I have two transports where I should have one. The function fails, because it tries to set the protocol's transport twice:

await loop.connect_read_pipe(lambda: protocol, sys.stdin)
await loop.connect_write_pipe(lambda: protocol, sys.stdout)
# !!!! assert self._transport is None, 'Transport already set'

I tried to pass a dummy protocol to the first line, but this line is not correct either, because both transports are needed, not just one:

writer = asyncio.StreamWriter(w_transport, protocol, reader, loop)

I guess I need to combine two unidirectional transports to one bidirectional somehow. Or is my approach entirely wrong? Could you please give me some advice?

---

UPDATE: after some test this seems to work (but does not look good to me):

async def connect_stdin_stdout(limit=_DEFAULT_LIMIT, loop=None):
    if loop is None:
        loop = asyncio.get_event_loop()
    reader = asyncio.StreamReader(limit=limit, loop=loop)
    protocol = asyncio.StreamReaderProtocol(reader, loop=loop)
    dummy = asyncio.Protocol()
    await loop.connect_read_pipe(lambda: protocol, sys.stdin) # sets read_transport
    w_transport, _ = await loop.connect_write_pipe(lambda: dummy, sys.stdout)
    writer = asyncio.StreamWriter(w_transport, protocol, reader, loop)
return reader, writer

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