Casted type from `typing.cast()` is ignored (`no-member`)

[postlund]

Steps to reproduce

Given a file a.py:

import wave
from typing import cast

with wave.open("dummy.wav", "wb") as handle:
    writer = cast(wave.Wave_write, handle)
    writer.setnchannels(2)
    writer.setsampwidth(2)
    writer.setframerate(44100)

Current behavior

Result of pylint a.py:

a.py:7:4: E1101: Instance of 'Wave_read' has no 'setnchannels' member; maybe 'getnchannels'? (no-member)
a.py:8:4: E1101: Instance of 'Wave_read' has no 'setsampwidth' member; maybe 'getsampwidth'? (no-member)
a.py:9:4: E1101: Instance of 'Wave_read' has no 'setframerate' member; maybe 'getframerate'? (no-member)

Expected behavior

No warnings expected. mypy does not give any errors.

pylint --version output

Result of pylint --version output:

pylint 3.0.0a3
astroid 2.5.7
Python 3.8.1 (default, Jan  8 2020, 13:13:59) 
[GCC 4.9.2]

[hippo91]

@postlund thanks for the report. Could you try without the call to cast? It would determine if the problem is linked to the inference of cast function or to the wave module.

[postlund]

@hippo91 I get the same error if I remove the cast by the looks of it.

[timmartin]

It looks to me like there's no logic currently to infer the result of the cast() function. IIUC this is something that should be solved in Astroid?

I had a go at implementing this here, I think this will need a bit of work to make it more rigorous but it seems like it works in a simple case.

[Pierre-Sassoulas]

@timmartin, could you open a PR in astroid please ? Apparently bad inference in cast is not at fault for this issue, but this is still a nice feature to have :)

[timmartin]

I will open a PR, I just wanted to do a little more review of the code first, it's probably not ready to merge.

If I'm understanding this correctly, lack of inference on typing.cast is causing this bug. It looks like if the cast was inferred correctly, Pylint would know to treat the writer as an instance of wave.Wave_write, which would presumably then have the expected methods.

[jacobtylerwalls]

See discussion at pylint-dev/astroid#1076 (comment) for subtle issues with TypeVar that a proposal would need to address.