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.

Beginne mit der Lektüre von PEP 484, der Python-Typing-Dokumentation und der mypy-Dokumentation zu den gezeigten Zuweisungen von Subklassen. Vergleiche die von mypy ausgegebenen Typen mit den Meldungen von Pylance und dokumentiere anschließend das beabsichtigte Verhalten oder kläre, ob die Abweichung als Fehler behandelt werden sollte.