Bug with explicit Mapping hints?

[dbushong]

from typing import Mapping

x: Mapping[int, int | str] = {1: 2}

reveal_type(x)

x[3] = "four"

In this example code, no error is produced on line 3, but on line 5 the type of x is given as dict[int, int]. On line 7 this is underscored when you finally get an error:

Argument of type "Literal['four']" cannot be assigned to parameter "value" of type "int" in function "__setitem__"
  "Literal['four']" is not assignable to "int"  (reportArgumentType)

See: https://pyright-play.net/?pythonVersion=3.14&strict=true&code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCyAhgsugFDkAeAXISWWgNqowA0mK%2BAPlAM4wQAXSgBeKAG8AjHQBMAX0ogApgDdlRADYB9eAmUAKKgEpKVJgGYR4gETAwAVxA2gA

It seems to me that either:

1. The : Mapping[...] type hint on line 3 should work, and the type of x should be Mapping[int, int | str] and no assignments should be allowed or
2. An error should be emitted on line 3 saying that it isn't allowed

[erictraut]

This is related to a recent discussion on the typing forum. That discussion involved a bunch of (unrelated) issues, but it raised the same question you're raising above.

The question is whether bidirectional type inference for the expression {1: 2} should result in dict[int, int] or dict[int, int | str]. Mypy produces the latter whereas pyright produces the former.

I need to give this some additional thought, but I'm not convinced that pyright's behavior is correct in this case.

[dbushong]

Iiiiinteresting. Thanks! I'll follow the discussion with interest.