Bug with filter

[heckad]

from typing import Optional
from typing import Sequence

from more_itertools import first


class A:
    def some_method(self):
        pass


def f(inp: Sequence[A]) -> Optional[A]:
    return first(filter(lambda a: a.some_method(), inp), None)

Output

test.py:13: error: Item "None" of "Optional[A]" has no attribute "some_method"

[msullivan]

Sigh, yet another inference context issue. Since inference involves a lot of guess work I'm not totally sure if this counts as a bug, but it is frustrating.

The issue seems to be that mypy is inferring from the return type and the call to first that filters return type ought to be Iterable[Optional[A]]. Looking at first's type, though, this is a little unusual?

If you do

x = first(filter(lambda a: a.some_method(), inp), None)`
return x

it works, since it doesn't look at the Optional[A] it is supposed to be returning and so manages to infer it... Hrm.