python abstractmethod with another baseclass breaks abstract functionality

Question

Consider the following code example

import abc
class ABCtest(abc.ABC):
    @abc.abstractmethod
    def foo(self):
             


        

Answer 1

I asked a similar question and based on user2357112 supports Monicas linked bug report, I came up with this workaround (based on the suggestion from Xiang Zhang):

from abc import ABC, abstractmethod

class Base(ABC):
    @abstractmethod
    def foo(self):
        pass

    @abstractmethod
    def bar(self):
        pass

    def __new__(cls, *args, **kwargs):
        abstractmethods = getattr(cls, '__abstractmethods__', None)
        if abstractmethods:
            msg = "Can't instantiate abstract class {name} with abstract method{suffix} {methods}"
            suffix = 's' if len(abstractmethods) > 1 else ''
            raise TypeError(msg.format(name=cls.__name__, suffix=suffix, methods=', '.join(abstractmethods)))
        return super().__new__(cls, *args, **kwargs)

class Derived(Base, tuple):
    pass

Derived()

This raises TypeError: Can't instantiate abstract class Derived with abstract methods bar, foo, which is the original behaviour.