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Type narrowed to Never when using TypeIs with union of callables #18009

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Viicos opened this issue Oct 21, 2024 · 1 comment · May be fixed by #18193
Open

Type narrowed to Never when using TypeIs with union of callables #18009

Viicos opened this issue Oct 21, 2024 · 1 comment · May be fixed by #18193
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bug mypy got something wrong topic-type-narrowing Conditional type narrowing / binder

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@Viicos
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Viicos commented Oct 21, 2024

playground

from typing import Any, Callable
from typing_extensions import TypeIs, assert_type, assert_never


def is_single_callable(c: Callable[[], Any] | Callable[[int], Any]) -> TypeIs[Callable[[int], Any]]:
    ...


def test(c: Callable[[], Any] | Callable[[int], Any]):
    if is_single_callable(c):
        reveal_type(c)
    else:
        # mypy thinks this is unreachable
        a: int = 1

While c could be narrowed to Callable[[], Any] in the else block, it is currently narrowed to Never. For reference, see this discussion.

Your Environment

  • Mypy version used: 1.12.1
  • Mypy command-line flags: --warn-unreachable
  • Python version used: 3.12
@Viicos Viicos added the bug mypy got something wrong label Oct 21, 2024
@brianschubert
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It turns out this affects any union of a generic type (not just Callable). For example:

def is_int_list(c: list[str] | list[int]) -> TypeIs[list[int]]:
    ...

def foo(x: list[str] | list[int]) -> None:
    if is_int_list(x):
        reveal_type(x)  # N: Revealed type is "builtins.list[builtins.int]"
    else:
        reveal_type(x)  # E: Statement is unreachable
    reveal_type(x)      # N: Revealed type is "builtins.list[builtins.int]"

@brianschubert brianschubert added the topic-type-narrowing Conditional type narrowing / binder label Nov 26, 2024
@brianschubert brianschubert linked a pull request Nov 26, 2024 that will close this issue
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bug mypy got something wrong topic-type-narrowing Conditional type narrowing / binder
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Fix TypeIs negative narrowing of union of generics brianschubert/mypy
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@brianschubert @Viicos