Type hint asyncio.Process streams (stdin, stdout, stderr) when created with values other than default None

[makukha]

Is it possible to update asyncio stub files so that asyncio.Process.{stdin,stdout,stderr} are resolved as non-None by mypy?

Example:

from asyncio import create_subprocess_exec
from asyncio.subprocess import PIPE

async def main() -> None:
    proc = await create_subprocess_exec('/usr/bin/ls', stdout=PIPE)
    async for line in proc.stdout:
        print(line)

Typing error reported by mypy (desired behaviour is no error):

error: Item "None" of "StreamReader | None" has no attribute "__aiter__" (not async iterable)  [union-attr]

Current asyncio.Process declaration in typeshed/stdlib/asyncio/subprocess.pyi:

class Process:
    stdin: streams.StreamWriter | None
    stdout: streams.StreamReader | None
    stderr: streams.StreamReader | None

[srittau]

Considering that type variable defaults can now be used, it makes sense to me to make Process generic over the various streams. In that case, it seems possibly to me to make this work properly, but I haven't looked too closely at it.

[makukha]

Functions affected:

• asyncio.create_subprocess_exec()
• asyncio.create_subprocess_shell()
• asyncio.loop.subprocess_exec()
• asyncio.loop.subprocess_shell()

Is it true that if using Process[TIN, TOUT, TERR] generic, every function above will require 8 overload alternatives? If so, this solution is questionable, especially regarding that the same problem appears for subprocess module that is already full of overloads.

[makukha]

The same problem appears in subprocess module.

• for subprocess.CompletedProcess it was decided to ignore the | None branch
• for subprocess.Popen there's the same "false positive" error:

from subprocess import PIPE, Popen

def main() -> None:
    popen = Popen('/usr/bin/ls', stdout=PIPE)
    for line in popen.stdout:
        print(line)

error: Item "None" of "IO[bytes] | None" has no attribute "__iter__" (not iterable)  [union-attr]