Python 3 type hint for a factory method on a base class returning a child class instance

Question

Let\'s say I have two classes Base and Child with a factory method in Base. The factory method calls another classmethod which may be

Answer 1

It is indeed possible: the feature is called TypeVar with Generic Self (though this is slightly misleading because we're using this for a class method in this case). I believe it behaves roughly equivalently to the "CRTP" technique you linked to (though I'm not a C++ expert so can't say for certain).

In any case, you would declare your base and child classes like so:

from typing import TypeVar, Type, Tuple

T = TypeVar('T', bound='Base')

class Base:
    @classmethod
    def create(cls: Type[T], *args: Tuple[Any]) -> T: ...

class Child(Base):
    @classmethod
    def create(cls, *args: Tuple[Any]) -> 'Child': ...

Note that:

1. We don't need to make the class itself generic since we only need a generic function
2. Setting the TypeVar's bound to 'Base' is strictly speaking optional, but is probably a good idea: this way, the callers of your base class/subclasses will at least be able to call methods defined in the base class even if you don't know exactly which subclass you're dealing with.
3. We can omit the annotation on cls for the child definition.