python custom metaclass call method relation with classes

Question

I am trying to understand the relation in between python metaclass and class. I was trying to create singleton class and found this code

class SingleTon(type):
    def __call__(self, *args, **kwargs):
        if self._instances is None:
            self._instances = super(SingleTon, self).__call__(*args, **kwargs)
        return self._instances

class Counter:
    __metaclass__ = SingleTon
    _instances = None
    def __init__(self):
        self.count = 1
c = Counter()

my question here is how counter class object is getting created using metaclass. I know metaclass call method gets called whenever we create an object but the confusion is here what this code super(SingleTon, self).__call__(*args, **kwargs) does here. Please explain. It would be very appreciable.

Dimitris Fasarakis Hilliard · Accepted Answer · 2017-02-24 18:04:27Z

1

super will just forward the arguments to type.__call__ which is responsible for the class creation.

It's like calling super in a 'normal' class hierarchy only now, you're calling it in a metaclass. Since SingleTon is a subclass of type, that'll get called. In a class scenario, you'd (normally) forward calls to the base class object.

answered Feb 24, 2017 at 18:04

Dimitris Fasarakis Hilliard

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4 Comments

dheeraj saini Over a year ago

So, that means i can use type directly instead of super ??

Dimitris Fasarakis Hilliard Over a year ago

Yup, try it out! (pass self too, though). That beats the point of using super, though (you're hard-coding names).

dheeraj saini Over a year ago

Thank you very much jim. Please clear one more thing type(metaclass) method call is responsible for creating instance object. Then how also it creates the class ?

Dimitris Fasarakis Hilliard Over a year ago

@dheerajsaini That's a loaded question to be honest :-). To keep it as short as I can: type.__call__ handles both cases, depending on what is passed to it either type.__new__/__init__ (or mymetaclass.__new__/__init__ in general) is going to get called for classes or object.__new__/__init__ or (class.__new__/__init__) for instances.

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