The Barebones - Python

We all know that objects are created from classes. But can a class be created from a class? Yes it can; this is called a metaclass. They give us a great deal of power when it comes to changing the behavior of a class. Python 2.2 supports metaclasses. Peyton McCullough explains.

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By: Peyton McCullough

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December 13, 2005

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Metaclasses are created just like classes, except they are a subclass of type or another metaclass. Actually, type is itself a metaclass. There isn't any interesting syntax involved with their creation:

>>> class MetaClass ( type ):
pass

Now, to demonstrate how we can use our overly-primitive metaclass, let's create a class from it. There are two ways to do this. The first way is to create a class from our metaclass just as we would create an object from a normal class. However, we have to make a few changes, or we'll end up with this:

>>> A = MetaClass()

Traceback (most recent call last):
File "<pyshell#40>", line 1, in -toplevel-
A = MetaClass()
TypeError: type() takes 1 or 3 arguments

Unless you instruct them to do otherwise, metaclasses accept three arguments: the name of the class to be created, a tuple of the base classes involved and a dictionary containing the class's attributes:

>>> A = MetaClass ( 'A', (), {} )

>>> A

We can see our metaclass of our class like this:

>>> A.__class__

Similarly, we can check the metaclass of an object like so:

>>> z = A()

>>> z.__class__.__class__

Creating some attributes is not very complicated. Let's create a class with a few variables:

>>> B = MetaClass ( 'B', (), { 'p': 4, 'q': ( '1', 2, '3' ), 'r': "%" } )

>>> B.p

>>> B.q

('1', 2, '3')

>>> B.r

'%'

Of course, this is pretty annoying and pointless for anything complex, which leads us to the second method of creating classes from metaclasses. You can simply assign the metaclass to a new-style class's __metaclass__ variable:

>>> class C ( object ):
__metaclass__ = MetaClass

This is a more practical method of creating classes from metaclasses than the previous method.

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