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]

    @property
    def instance(cls):
        return cls()


# Usage
class Foo(metaclass=Singleton):
    def __init__(self):
        self.foo = 1


class Bar(metaclass=Singleton):
    def __init__(self):
        self.bar = 2


print(Foo.instance.foo)
print(Bar.instance.bar)

Foo.instance.foo += 10
Bar.instance.bar += 20

print(Foo.instance.foo)
print(Bar.instance.bar)

Foo().foo += 10
Bar().bar += 20

print(Foo().foo)
print(Bar().bar)