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TypeAlias does not work when attribute-accessed from the typing_extensions package #12179

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bzoracler opened this issue Feb 15, 2022 · 1 comment · Fixed by #12180
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TypeAlias does not work when attribute-accessed from the typing_extensions package #12179

bzoracler opened this issue Feb 15, 2022 · 1 comment · Fixed by #12180
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bug mypy got something wrong

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@bzoracler
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Bug Report

TypeAlias does not trigger errors in mypy if accessed from the typing_extensions package (rather than being imported directly).

To Reproduce

import typing as tp
import typing_extensions as tpx
from typing_extensions import TypeAlias

ASDF: TypeAlias = tp.Literal["A", "S", "D", "F"]
JKL: tpx.TypeAlias = tp.Literal["J", "K", "L"]

Int: tpx.TypeAlias = int
Str: TypeAlias = str

Expected Behavior

Everything below should throw errors in mypy:

G: ASDF = "G"  # error: Incompatible types in assignment (expression has type "Literal['G']", variable has type "Union[Literal['A'], Literal['S'], Literal['D'], Literal['F']]")
H: JKL = "H"  # error: Incompatible types in assignment (expression has type "Literal['H']", variable has type "Union[Literal['J'], Literal['K'], Literal['L']]")
n: Int = "9"  # error: Incompatible types in assignment (expression has type "str", variable has type "int")
s: Str = 9  # error: Incompatible types in assignment (expression has type "int", variable has type "str")

Actual Behavior

The aliases JKL and Int, typed using tpx.TypeAlias, do not trigger errors with mypy:

G: ASDF = "G"  # error: Incompatible types in assignment (expression has type "Literal['G']", variable has type "Union[Literal['A'], Literal['S'], Literal['D'], Literal['F']]")
H: JKL = "H"  # Expected error here; got none
n: Int = "9"  # Expected error here; got none
s: Str = 9  # error: Incompatible types in assignment (expression has type "int", variable has type "str")

Your Environment

  • Mypy version used: 0.931
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): 
    [mypy]
plugins = numpy.typing.mypy_plugin
ignore_missing_imports = True
  • Python version used: 3.8.12
  • Operating system and version: Elementary OS 6.1 Jólnir (Based on Ubuntu 20.04 LTS)
@bzoracler bzoracler added the bug mypy got something wrong label Feb 15, 2022
@hauntsaninja
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hauntsaninja commented Feb 15, 2022 •
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Yikes, this is pretty bad. Thanks for the report. I think the issue isn't about typing_extensions vs typing (which tests cover), but rather the import alias. That is, tp.TypeAlias fails as well. Looks easy to fix, though!

hauntsaninja pushed a commit to hauntsaninja/mypy that referenced this issue Feb 15, 2022
Fix use of TypeAlias from aliased imports
08ee2a1 
Fixes python#12179
@hauntsaninja hauntsaninja mentioned this issue Feb 15, 2022
Merged
JelleZijlstra pushed a commit that referenced this issue Feb 15, 2022
@hauntsaninja
Fix use of TypeAlias from aliased imports (#12180)
cd3dfbd 
Fixes #12179
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Fix use of TypeAlias from aliased imports hauntsaninja/mypy
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