Inherit a generic type in python 3 with typing

Question

I am doing some input experiments in Python 3.6. I want to create an entity class that can be created in two ways:

• Using a regular initializer ( p = Person(name='Hannes', age=27) )
• Statically from a state object ( p = Person.from_state(person_state) ).

The Entity class from which Person derives has a state class as a generic parameter. However, when checking the code with pypy, I get an error that Person # from_state is not getting the state type from the class that it inherits:

untitled2.py:47: error: argument 1 "from_state" in "Entity" has an incompatible type "UserState"; expected "StateType"

I thought that inheriting from Entity[UserState] , StateType would be bound to UserState , and method signatures in child classes would be updated accordingly.

This is the complete code. I marked a line where, as I suspect, I am doing something wrong with ????? . Line 47 is almost at the bottom and marked in the code.

 from typing import TypeVar, Generic, NamedTuple, List, NewType EntityId = NewType('EntityId', str) StateType = TypeVar('StateType') class Entity(Generic[StateType]): id: EntityId = None state: StateType = None @classmethod def from_state(cls, state: StateType): # ????? ret = object.__new__(cls) ret.id = None ret.state = state return ret def assign_id(self, id: EntityId) -> None: self.id = id class UserState(NamedTuple): name: str age: int class User(Entity[UserState]): def __init__(self, name, age) -> None: super().__init__() self.state = UserState(name=name, age=age) @property def name(self) -> str: return self.state.name @property def age(self) -> int: return self.state.age def have_birthday(self) -> None: new_age = self.state.age+1 self.state = self.state._replace(age=new_age) # Create first object with constructor u1 = User(name='Anders', age=47) # Create second object from state user_state = UserState(name='Hannes', age=27) u2 = User.from_state(user_state) # Line 47 print(u1.state) print(u2.state)