child class as singleton

In python, I want to design two classes :

1. class A which will have a set of methods.
2. class B which will have all the method of A and be a singleton.

I wrote it like this :

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


class A:
    def __init__(self, toto):
        self.toto = toto

    def another_method(self):
        ...


class B(A, metaclass=Singleton):
    pass

is it the proper way to define A and B ?