Python3 Singleton metaclass method not working

问题

I saw a lot of methods of making a singleton in Python and I tried to use the metaclass implementation with Python 3.2 (Windows), but it doesn"t seem to return the same instance of my singleton class.

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 MyClass(object):
    __metaclass__ = Singleton

a = MyClass()
b = MyClass()
print(a is b) # False

I use the decorator implementation now which is working, but I'm wondering what is wrong with this implementation?

回答1:

The metaclass syntax has changed in Python3. See the documentaition.

class MyClass(metaclass=Singleton):
    pass

And it works:

>>> MyClass() is MyClass()
True

来源：https://stackoverflow.com/questions/17237857/python3-singleton-metaclass-method-not-working