`Derived` does not implement `method`

What it does

Checks for @final classes that have unimplemented abstract methods.

Why is this bad?

A class decorated with @final cannot be subclassed. If such a class has abstract methods that are not implemented, the class can never be properly instantiated, as the abstract methods can never be implemented (since subclassing is prohibited).

At runtime, instantiation of classes with unimplemented abstract methods is only prevented for classes that have ABCMeta (or a subclass of it) as their metaclass. However, type checkers also enforce this for classes that do not use ABCMeta, since the intent for the class to be abstract is clear from the use of @abstractmethod.

Example

from abc import ABC, abstractmethod
from typing import final


class Base(ABC):
    @abstractmethod
    def method(self) -> int: ...


@final
# `Derived` does not implement `method`
class Derived(Base):  # error
    pass