Add __init__ to subprocess.Popen

[actionless]

• Are you reporting a bug, or opening a feature request?
  Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

import subprocess

class MyPopen(subprocess.Popen):

    def __init__(self) -> None:
        args = ['echo', 'test']
        super().__init__(
            args=args,
        )

• What is the actual behavior/output?

$ python -m mypy mypy_subprocess.py
mypy_subprocess.py:9: error: Unexpected keyword argument "args" for "__init__" of "object"
Found 1 error in 1 file (checked 1 source file)

• What are the versions of mypy and Python you are using?

$ python -m mypy --version
mypy 0.740+dev.742d33a41271206d6eb7dd2e4b04659de4683597

$ python --version
Python 3.7.4

[actionless]

NOTE: it was working fine with mypy==0.720

[ilevkivskyi]

This is a subtle problem in typeshed. @msullivan this is a one little additional argument in favour of supporting overloading on self in __init__() I proposed earlier, not that it is something we should do right now, but still.

I think it however makes sense to add a dummy __init__() to suppress this (if this will not cause other troubles). Also potentially we can move this issue to typeshed.

[ilevkivskyi]

I think this actually belongs in typeshed, I will try the new Github transfer issue feature now.

[JelleZijlstra]

Renamed the issue to reflect what's actually being asked for.