forbid-children-method-extending.py

class Parent
    ...

    @classmethod
    def __init_subclass__(cls, **kwargs):
        dir_subclass = set(field for field in dir(cls) if not field.startswith(f"_{cls.__name__}__"))
        dir_base = set(field for field in dir(cls.__base__))
        extra_dir = dir_subclass.difference(dir_base)
        if extra_dir:
            raise CellCantHasExtraFieldException(
                f"Нельзя расширять класс. Дополнительные поля: {extra_dir}"
            )
        return super().__init_subclass__(**kwargs)