[Python] Typing error with inherited classes having overloaded constructor with different...

With the code below:

import abc

class ABCParent(metaclass=abc.ABCMeta):
def __init__(self, a: str, sibling_type: type[ABCParent]) -> None:
self.a = a
self._sibling_type = sibling_type
def new_sibling(self, a: str) -> ABCParent:
return self._sibling_type(a)

class ChildA(ABCParent):
def __init__(self, a: str) -> None:
super().__init__(a, ChildB)

class ChildB(ABCParent):
def __init__(self, a: str) -> None:
super().__init__(a, ChildA)


I get the following typing error, using mypy in strict mode.

src/demo_problem.py: note: In member "new_sibling" of class "ABCParent":
src/demo_problem.py:10:16:10:36: error: Missing positional argument "sibling_type" in call to "ABCParent" [call-arg]
return self._sibling_type(a)


This is perfectly logical, since ABCParent requests the sibling_type argument. But in my case, the self._sibling_type will be one of the child classes, which override the constructor such that sibling_type is not needed.

I was able to work around the problem by using a typing.Union of the child classes:

import abc
import typing

TConcreteChild: typing.TypeAlias = typing.Union["ChildA", "ChildB"]

class ABCParent(metaclass=abc.ABCMeta):
def __init__(self, a: str, sibling_type: type[TConcreteChild]) -> None:
...


But I don't like this because:

• It involves defining a type alias (TConcreteChild) which is otherwise useless
• The definition of this alias must be updated if more child classes are defined
• It involves defining types before definitions of the classes, so it requires forward references using strings (and therefore it can't use the new ChildA | ChildB syntax).

How can I do this more cleanly, while still having strict type checking?

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