python / python/typeshed

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

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stubs: false positive
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Description

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

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the Process declaration in typeshed/stdlib/asyncio/subprocess.pyi and trace the asyncio subprocess declarations used by create_subprocess_exec. Reproduce the mypy error from the issue's example, then verify that passing PIPE makes the relevant stream attributes non-optional without changing the default None behavior.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
developer-experience
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
42/100

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