(🎁) Support for/else statement in partially defined check

[KotlinIsland]

l: list[int]
for _ in l:
    pass
else:
    result = False
print(result)  # error: Name "result" may be undefined  [partially-defined]

This is incorrect, result will always be defined in this case.

[ilinum]

Thanks for the bug report!

mypy isn't smart enough to detect that the list l is always empty. Note that technically the body of the loop is unreachable but it's not flagged with --warn-unreachable (mypy-play).

Do you have a real-world example where this would matter? In the toy example here it seems like it's better to just simplify everything to result = False and remove the loop.

[KotlinIsland]

mypy isn't smart enough to detect that the list l is always empty.

This issue isn't regarding an empty list, the else branch will always be executed here regardless of if l has items or not, and result will always be set to False

Note that technically the body of the loop is unreachable

Again, this isn't regarding a list that is empty, so the body is potentially reachable.

Do you have a real-world example where this would matter?

Yes, some projects I have lit up with false positives when I updated mypy. Basically any scenario where a for/else is used to set a fallback value.

[ilinum]

Yep! You're totally right. Thanks for pointing this out; sent a PR with the fix.

[KotlinIsland]

follow up: #14209