CODEWITHSUNDEEP
pythonoop
crazy_rudy
crazy_rudy

Reputation: 157

Why do metaclass have a type?

I've little bit test to fully understand metaclass in python.

class Test(object):
    pass


print Test.__class__
print Test.__class__.__class__
print Test.__class__.__class__.__class__

All of result is same type. but each of their address is not same
I can't really understand why metaclass has a metaclass recursively.
Explain me please?

Upvotes: 1

Views: 143

Answers (4)

Erxin
Erxin

Reputation: 1856

All the python's class object is build by the built-in function type(). You could also try this.

>>> T.__class__ == type
True
>>> type(type)
<type 'type'>

The T.class is equal to the build-in function type which is also an object implemented the call function. It's a attribute will be interpret as class(T). As your T class have no base class so type() is used which will return the type object.

You could check the python doc about customizing class creation to get detail about class creation.

To determining the appropriate metaclass

  • if no bases and no explicit metaclass are given, then type() is used
  • if an explicit metaclass is given and it is not an instance of type(), then it is used directly as the metaclass
  • if an instance of type() is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used

Upvotes: 0

Roman Bodnarchuk
Roman Bodnarchuk

Reputation: 29727

Actually, addresses are the same:

>>> id(Test.__class__)
6384576
>>> id(Test.__class__.__class__)
6384576
>>> id(Test.__class__.__class__.__class__)
6384576

Everything is an object in Python, and each object must have a class (it should belong to some type). You can access that class/type reference by __class__ attribute, e.g.:

>>> (1).__class__
<type 'int'>

Everything includes classes itself, which are of class/type called type:

>>> (1).__class__.__class__
<type 'type'>

In the same time type 'type'> is also an object and should reference to some class/type. But since this is kind of special object, its __class__ attribute refers to itself:

>>> (1).__class__.__class__.__class__ is (1).__class__.__class__
True

Upvotes: 2

Steve Barnes
Steve Barnes

Reputation: 28405

All classes are classes which means they are derived from a class called class...

Upvotes: 0

pascalhein
pascalhein

Reputation: 5846

When you do Test.__class__, you get back the type of Test, which is type (because Test is a class identifier).

type itself is again a class identifier, so you can call __class__ (which is inherited from object) on it and you get back that its type is, again, type because it is a class identifier.

Because you will always get back type which is a class itself, you can do this infinitely many times and will always get back that the current object's type is type.

Upvotes: 0

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