Getting this typing error:

error: Incompatible types in assignment (expression has type "object", variable has type "A | B")  [assignment]

With this code:

from dataclasses import dataclass
from typing import TypeVar, Mapping, reveal_type


@dataclass
class A:
    foo: str = "a"

@dataclass
class B:
    bar: str = "b"

lookup_table: Mapping[str, type[A] | type[B]] = {
    "a": A,
    "b": B
}

reveal_type(lookup_table)  # note: Revealed type is "typing.Mapping[builtins.str, Union[type[simple.A], type[simple.B]]]"
T = TypeVar("T")

def load(lookup_table: Mapping[str, type[T]], lookup_key:str) -> T:
    con: type[T] = lookup_table[lookup_key]
    instance: T = con()
    return instance

example_a: A | B = load(lookup_table, "a")  # error: Incompatible types in assignment (expression has type "object", variable has type "A | B")
print(example_a)

Edit: Logged a mypy bug here: https://github.com/python/mypy/issues/18265

解决方案

The error occurs because the load function is returning either (A|B) which is not detected by the type checker in that function, as it was expecting an object . Therefore, it is giving an error of incompatible types.