Casting an attribute

[amacfie-tc]

from typing import Any, cast, reveal_type


class C:
    a: dict[int, int] | None = None


c = C()
c.a = cast(dict[int, Any], c.a)
reveal_type(c.a)  # Type of "c.a" is "dict[int, int]"

What can a cast call be assigned to for Pyright to understand? Just atomic names like foo?

[erictraut]

A cast doesn't affect assignment. The subexpression cast(dict[int, Any], c.a) simply tells a type checker to ignore the type it normally evaluates for the c.a and replace that type with dict[int, Any].

When you assign a value of type dict[int, Any] (whether that comes from normal type evaluation or a cast) to an attribute whose type is declared as dict[int, int] | None, the assignment is allowed because dict[int, Any] is assignable to dict[int, int] | None. Type narrowing is applied on assignment. The type dict[int, int] | None is narrowed to dict[int, int] in this case. The reveal_type call displays the locally-narrowed type at that location within the program.