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type checking in lambdas stops working when using Iterable instead of list #20163
Description
Say we have a function like this:
from collections.abc import Callable, Iterable
def f[T](a: Callable[[T], Iterable[list[T]]]) -> None: ...This generates no errors:
from typing import assert_never
f(lambda a: [assert_never(True)])It would appear there's no checking in the lambda body.
However if I change the Iterable to list:
def f[T](a: Callable[[T], list[list[T]]]) -> None: ...Then it works, and an error is shown as expected.
Activity
A5rocks commented on Nov 2, 2025
For some reason this fails:
with:
Maybe there's something that's hiding the error messages (maybe we eagerly evaluate for type context and don't want possibly wrong output?) but then we're forgetting to actually show the error messages later?
Do not cache fast container types inside lambdas (#20166)