metaclass in python

[Humble]

Whenever you create a class in python you would have noticed its of type “type”.

>>> class A(object):
...     def __init__(self):
...             print "I am a constrcutor"
... 
>>> type(A)
<type 'type'>
>>> A
<class '__main__.A'>
>>> 



In simple terms, class A is created from "type" or in other way "type" is a metaclass of class A.

As like we create objects from class , class is an object of "type".

In other way, an object is an instance of its class; like that, a class is an instance of its metaclass or we would say class of a class is its metaclass.


In one of the earlier posts ( ) I mentioned how to create a class using "type".. 


instead of using "type" as your metaclass, you can use your own custom metaclass. If you remember, when we discussed @abstractmethod decorator we used abc.ABCMeta as our metaclass. It was achived by 

__metaclass__ = ABCMeta

in the class definition. 

When we define our own metaclass, we inherit it from builtin "type" and manipulate __new__ method ( where the arguments are first passed from new class creation) of custom metaclass. Then return to builtin "type"'s __new__ method with the essential arguments.

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