I have a data class having as member a list of objects of another class having as member an object of type Optional[CustomType], and I need to call a method of CustomType class on that member, only when the member is not None.
Even if such check is explicitly performed, mypy still complains about the missing attribute of the item None of the CustomType | None union.

Here follows a working example using str as CustomType that highlights the issue:

from dataclasses import dataclass
from typing import Optional


@dataclass(frozen=True)
class Sample:
    tag: Optional[str] = None


@dataclass(frozen=True)
class Container:
    samples: list[Sample]



if __name__ == "__main__":

    containers: list[Container] = []
    tags: list[str] = []

    for container in containers:
        for index in range(len(container.samples)):
            if container.samples[index].tag is not None:
                tags.append(container.samples[index].tag.capitalize())

This gives Item "None" of "str | None" has no attribute "capitalize" Mypyunion-attr error on mypy.

I’ve discovered that the issue vanishes if one avoids indexing and iterates over the elements directly (which I may not)

    for container in containers:
        for sample in container.samples:
            if sample.tag is not None:
                tags.append(sample.tag.capitalize())

so it seems that mypy struggles resolving the type because of the nested structure.

Other workarounds which I cannot use involve asserting or assigning the value to a temporary variable for checking. The walrus operator assignment would do as well:

    for container in containers:
        for index in range(len(container.samples)):
            if (tag := container.samples[index].tag) is not None:
                tags.append(tag.capitalize())

but apparently cannot be used in a more complex context (Operator ":=" is not allowed in this context without surrounding parentheses).

What am I missing here?