The general problem is that mypy doesn't really understand that a descriptor on an attribute is going to have the return value of __get__ not __call__/__init__

descriptor_typing_problem.py

from enum import Enum
from typing import Type, Any

Sentinels = Enum("Sentinels", "NO_INIT")

class attr_descriptor:

    def __init__(self, default=Sentinels.NO_INIT):
        self.default = default

    def __set_name__(self, owner, name):
        self.owner = owner
        self.name = name
        if hasattr(owner, "__annotations__"):
            self.value_type = owner.__annotations__.get(name, None)
        self.value_type = Sentinels.NO_INIT

    def __get__(self, instance: 'BaseClass', typevar: Type['BaseClass']) -> Any:
        if instance is None:
            return self
        if self.name in instance.__dict__:
            return instance.__dict__[self.name]
        if self.default != Sentinels.NO_INIT:
            return self.default
        if self.value_type != Sentinels.NO_INIT:
            return self.value_type()
        return None

class BaseClass:
    thing: int = attr_descriptor(default=10)
    def __init__(self, thing):
        if thing:
            self.thing = thing

BaseClass(thing=2)