In python, How do I create a abstract base singleton class that can only be initialized once?

Question

I have this piece of code

import abc


class SingletonABCMeta(abc.ABCMeta):
    _instances = {}

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


class Cabc(abc.ABC):
    __metaclass__ = SingletonABCMeta

    @abc.abstractmethod
    def foo(self): pass


class C(Cabc):

    def __init__(self, a):
        print(f"initializing with {a}")
        self._a = a

    def foo(self):
        return self._a


class DoubleC(Cabc):

    def __init__(self, a):
        print(f"initializing with {a}")
        self._a = a

    def foo(self):
        return self._a * 2


if __name__ == "__main__":
    print(C(2).foo())
    print(DoubleC(2).foo())

    print(C(3).foo())
    print(DoubleC(3).foo())

it returns

initializing with 2
2
initializing with 2
4
initializing with 3
3
initializing with 3
6

But I expect it to return

initializing with 2
2
initializing with 2
4
2
4

How do I do this?

Answer 1

figured it out

I changed from using __metaclass__ = SingletonABCMeta to the python3 version of it that is Cabc(abc.ABC, metaclass=SingletonABCMeta)

import abc


class SingletonABCMeta(abc.ABCMeta):
    _instances = {}

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


class Cabc(abc.ABC, metaclass=SingletonABCMeta):

    @abc.abstractmethod
    def foo(self): pass


class C(Cabc):

    def __init__(self, a):
        print(f"initializing with {a}")
        self._a = a

    def foo(self):
        return self._a


if __name__ == "__main__":
    print(C(2).foo())
    print(C(3).foo())

returns:

initializing with 2
2
2

But now that I think I should raise an exception if I do not want user to create a new instance of this class. Then I am going with this solution

import abc


class SingletonABCMeta(abc.ABCMeta):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(SingletonABCMeta, cls).__call__(*args, **kwargs)
            return cls._instances[cls]
        else:
            raise Exception("This is a singleton. no more than one instance of it can exist")


class Cabc(abc.ABC, metaclass=SingletonABCMeta):

    @abc.abstractmethod
    def foo(self): pass


class C(Cabc):

    def __init__(self, a):
        print(f"initializing with {a}")
        self._a = a

    def foo(self):
        return self._a


if __name__ == "__main__":
    print(C(2).foo())
    print(C(3).foo())