上下文

我开始用python探索元类的概念。很快，我发现自己面临着一个共同的问题。 TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases 根据我的理解，当您创建一个继承自两个（或更多）类[a，b]的类[c]时，就会发生这种情况，而这两个类不共享同一个元类[m_a，m_b]

M_A     M_B
 :       :
 :       :
 A       B
  \     /
   \   /
     C

这里很好地描述了这个问题，解决方案也很简单。我们需要创建一个新的元类[m_c]，它继承自m_a和m_b
我试图通过创建一个动态创建[m_c]的方法，使这个过程自动化。像这样的

我的问题

我的类[c]继承自[b]，我想使用[m_a]有它的元类，
m_a是一个自定义元类（singleton）
b的元类是.meta
我的 metaclass_resolver() 成功创建新元类[m_c]
但是它继承自 .ABCMeta 及 type 而不是继承自 .ABCMeta 及 M_A .

from collections import UserDict

def metaclass_resolver(*classes):

    metaclasses     = tuple(set(type(cls) for cls in classes))
    new_metaclass   = metaclasses[0]
    new_meta_name   = new_metaclass.__name__

    #if there's more than one metaclass
    #combine them and create a new metaclass (M_C)
    if len(metaclasses) > 1:
        #get the name of each metaclass
        new_meta_name = "_".join(mcls.__name__ for mcls in metaclasses)
        #create a new dynamic class
        #               type('name','bases {inheritance}, attrs)'
        new_metaclass = type(new_meta_name, metaclasses, {})

    return new_metaclass(new_meta_name, metaclasses, {})

# my custom metaclass (singleton)

class M_A(type):
    def __new__(cls, name, bases, attrs):
        c = super().__new__(cls, name, bases, attrs)
        return c

    def __init__(cls, name, bases, attrs):
        #When this metaclass is initiated, no instance of the class
        #using this metaclass would have been created
        cls.__instance = None
        super().__init__(name, bases, attrs)

    def __call__(cls, *args,**kwargs):
        #get the saved instance
        instance = cls.__instance
        #if the instance does not exists
        if instance is None:
            #create one
            instance = cls.__new__(cls)
            instance.__init__(*args,**kwargs)
            #save it
            cls.__instance = instance

        return instance
    pass

# abc metaclass

class B(UserDict):
    def method_needed_in_C(self):
        pass

# my class

class C(B, metaclass = metaclass_resolver(M_A, B)):
    def __init__(self):
        super().__init__()
        print(type(self.__class__))
        #<class 'abc.ABCMeta_type'>
    pass

if __name__ == "__main__":
    c = C()

在里面 metaclass_resolver() 当我使用 type(cls) 它回来了 <class 'type'> 而不是 <class 'M_A'> 因为每个类都是从 type . 但是，我怎样才能直接指出 <class 'M_A'> ?
如果我使用 cls 我直接得到这个错误： TypeError: descriptor '__init__' requires a 'type' object but received a 'str' 谢谢