Use any and all to make collection non-optional

[ilevkivskyi]

I have seen patterns like this several times:

def func() -> Optional[int]:
    pass

def test() -> List[int]:
    vals: Iterable[Optional[int]] = [func() for _ in some_list]
    if all(vals):
        return list(vals)  # None can't be in 'vals' here.
    return []

It would be convenient if binder could understand this for covariant collections.

[gvanrossum]

Clever and convenient, but we'd have to be sure we are okay with the refined type of `vals` not being a subtype of its original type (because of List invariance).

[ilevkivskyi]

but we'd have to be sure we are okay with the
refined type of vals not being a subtype of its original type (because of
List invariance).

I think we should only allow narrowing if the original variable is annotated with something like Iterable, Sequence or Container. Invariant containers like List or Dict should stay as is (I think there is already somer code in binder that applies a restriction only if it is a subtype).

[gvanrossum]

Then your example won't work as written.

Also, I have a vague sense of "déjà vu" about this.

[ilevkivskyi]

Then your example won't work as written.

OK, I will updated it.

Also, I have a vague sense of "déjà vu" about this.

Because I proposed and implemented a similar feature for if None not in some_container.

[msullivan]

I think if we only do this for covariant containers then it feels pretty niche and if we do it for invariant containers then it has problems, so I'm going to close this, but if somebody disagrees with me feel free to reopen