Type narrowing after use of all() built-in

[FinemechanicPub]

I think it would be great if Pyright could detect that a sequence can't contain None after a check with all():

from typing import Sequence


def func(nums: Sequence[int | None]):
    if not all(nums):
        raise ValueError
    total = 0
    for num in nums:
        total += num
    return total

[erictraut]

This isn't currently a supported type guard pattern. For a full list of supported type guard patterns, refer to this documentation.

I've seen the all(x) form used in real-world code, but the type of x can rarely be narrowed because it's most often an invariant container type like list or set. Your code example uses a Sequence, which is covariant. That means it could work in this case; it would be safe to narrow Sequence[int | None] to Sequence[int]. But I'm not convinced that adding the special support for this pattern is worth it because it wouldn't work very often in real code.

I recommend using a user-defined type guard (with the TypeIs special form) to handle your use case.

from typing import Sequence, TypeIs

def no_nones_or_zeros[T](val: Sequence[T | None]) -> TypeIs[Sequence[T]]:
    return all(val)

def func(nums: Sequence[int | None]):
    if not no_nones_or_zeros(nums):
        raise ValueError
    total = 0
    for num in nums:
        total += num
    return total

[FinemechanicPub]

Thank you for advice.