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.

Comienza leyendo PEP 484, la documentación de typing de Python y la documentación de mypy sobre las asignaciones de subclases mostradas. Compara los tipos revelados por mypy con los informes de Pylance y, después, documenta el comportamiento previsto o aclara si la discrepancia debe tratarse como un error.