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issubclass doesn't narrow down types #10978

@Illviljan

Description

@Illviljan

Bug Report

issubclass appears to not narrow down possible types.

To Reproduce

from typing import Any, Mapping, MutableMapping


def test_mutablemapping(obj: Mapping[Any, Any]) -> MutableMapping[Any, Any]:
    if issubclass(type(obj), MutableMapping):
        return obj  # error: Incompatible types in assignment (expression has type "Mapping[Any, Any]", variable has type "MutableMapping[Any, Any]")
    else:
        return dict(obj)

Expected Behavior

To narrow down in similar fashion to isinstance:

from typing import Any, Mapping, MutableMapping


def test_mutablemapping2(obj: Mapping[Any, Any]) -> MutableMapping[Any, Any]:
    if isinstance(obj, MutableMapping):
        return obj
    else:
        return dict(obj)

Your Environment

  • Mypy version used: 0.910
  • Mypy command-line flags: default
  • Mypy configuration options from mypy.ini (and other config files): default
  • Python version used: 3.8.8
  • Operating system and version: Windows 10

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    bugmypy got something wrongtopic-type-narrowingConditional type narrowing / binder

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