Subclass attribute type narrowing on assignment

Documentation

Consider this code snippet:

from typing import Iterable


class A:
    attr1: Iterable[str]  # narrows to different type, specific number of elements.
    attr2: tuple[str, ...]  # narrows to specific number of elements



class B(A):
    attr1 = "a", "b"
    attr2 = "a", "b", "c"


reveal_type(B.attr1)
reveal_type(B.attr2)

With this, mypy will tell us that Revealed type is "Tuple[builtins.str, builtins.str]" and Revealed type is "Tuple[builtins.str, builtins.str, builtins.str]" for attr1 and attr2 respectively. I can't find this behaviour documented, neither in PEP484, nor the Python typing docs, nor mypy docs. I don't even know where this should be documented best. But for what it's worth, I asked pylance, which says Type of "B.attr1" is "tuple[str, ...]" and Type of "B.attr2" is "tuple[str, ...]". Which tells me that the intended behaviour isn't clear for other people, too.

Note: I'm aware that this may be a bug report instead. But I rather err on the side of this not being clearly specified yet than being incorrectly implemented.

Start by reading PEP 484, the Python typing documentation, and the mypy documentation for the subclass assignments shown. Compare mypy's revealed types with Pylance's reports, then document the intended behavior or clarify whether the discrepancy should be treated as a bug.