The implementation of the abstract class is judged by mypy to be unconstructable

Bug Report

Consider the following code:

#!/usr/bin/env python3

from abc import ABC, abstractmethod
from typing import override


class Base(ABC):
    def __init__(self, val: int):
        self.__val = val

    def show(self) -> None:
        print(f'hello: {self.__val}')

    @abstractmethod
    def impl(self) -> None:
        pass


class Derive1(Base):
    def __init__(self):
        super().__init__(1)

    @override
    def impl(self) -> None:
        pass


class Derive2(Base):
    def __init__(self):
        super().__init__(2)

    @override
    def impl(self) -> None:
        pass


def demo(index: int) -> Base:
    classes = [Derive1, Derive2]
    return classes[index]()


if __name__ == '__main__':
    demo(0).show()

It implements Derive1 and Derive2 on the Base abstract class, which implement the impl interface. When checking it with mypy, everything looks fine:

> mypy demo.py
Success: no issues found in 1 source file

At this time, if you try to add Derive3, the code is as follows:

#!/usr/bin/env python3

from abc import ABC, abstractmethod
from typing import override


class Base(ABC):
    def __init__(self, val: int):
        self.__val = val

    def show(self) -> None:
        print(f'hello: {self.__val}')

    @abstractmethod
    def impl(self) -> None:
        pass


class Derive1(Base):
    def __init__(self):
        super().__init__(1)

    @override
    def impl(self) -> None:
        pass


class Derive2(Base):
    def __init__(self):
        super().__init__(2)

    @override
    def impl(self) -> None:
        pass


class Derive3(Base):  # add Derive3 just like the previous two
    def __init__(self):
        super().__init__(3)

    @override
    def impl(self) -> None:
        pass


def demo(index: int) -> Base:
    classes = [Derive1, Derive2, Derive3]  # add Derive3 item
    return classes[index]()


if __name__ == '__main__':
    demo(0).show()

At this time, mypy will issue the following complaint:

> mypy demo.py
demo.py:48: error: Cannot instantiate abstract class "Base" with abstract attribute "impl"  [abstract]
Found 1 error in 1 file (checked 1 source file)

The strange thing is why there is no error when there are two implementations, but there will be an error when there are three or more.

I tried pyright and pycharm, and they both gave no error messages for the above two codes, and I didn't find any reports or explanations of the problem on the search engine.

Environment

• Mypy version used: 1.15.0
• Mypy command-line flags: mypy demo.py
• Python version used: 3.13.2

Beginne damit, die Beispiele in demo.py mit mypy 1.15.0 unter Python 3.13.2 auszuführen, und vergleiche die Listen mit zwei und drei Klassen. Verfolge, warum die inferierte Klassensammlung nur im letzteren Fall einen Fehler wegen der Instanziierung des abstrakten Base erzeugt. Als abgeschlossen gilt die Aufgabe, wenn das Verhalten erklärt ist und, falls es als Fehler bestätigt wird, ein Regressionstest die fälschlicherweise ausgegebene Diagnose verhindert, ohne echte Fehler bei abstrakten Klassen zu verbergen.