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Type alias for a union of two @runtime_checkable Protocols triggers "Cannot use parameterized generics in instance checks" #16707

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InSyncWithFoo opened this issue Dec 26, 2023 · 0 comments
Open

Type alias for a union of two @runtime_checkable Protocols triggers "Cannot use parameterized generics in instance checks" #16707

InSyncWithFoo opened this issue Dec 26, 2023 · 0 comments
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bug mypy got something wrong

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@InSyncWithFoo
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Minimal examples

Mypy says I'm using parameterized generics in instance checks, but that's incorrect (playground):

from typing import SupportsIndex, SupportsInt

_ConvertibleToInt = SupportsInt | SupportsIndex

def f(o: object) -> None:
    if isinstance(o, _ConvertibleToInt):  # error
        ...

The same error does not happen if there is no alias (playground):

def f(o: object) -> None:
    if isinstance(o, SupportsInt | SupportsIndex):  # fine
        ...

...or if the alias is for one Protocol and not a union (playground):

_ConvertibleToInt = SupportsInt

def f(o: object) -> None:
    if isinstance(o, _ConvertibleToInt):  # fine
        ...

A union of the same Protocol repeated twice still triggers this error (playground):

_ConvertibleToInt = SupportsInt | SupportsInt

def f(o: object) -> None:
    if isinstance(o, _ConvertibleToInt):  # error
        ...

Environment

  • Versions: 1.7 to 1.8
  • Flags: --strict
  • Python versions: 3.10, 3.11, 3.12
@InSyncWithFoo InSyncWithFoo added the bug mypy got something wrong label Dec 26, 2023
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