The magic of metaclasses in Python

Metaclasses are a way for you to have a class act as the template for another class. They are simlar to a classfactory, in that they create new classes. In words of Tim Peters Metaclasses are deeper magic than 99% of people are going to need.

However, in right hands they can be a potent tool, in particular Django uses metaclasses beautifully to create very beautiful declarative models. Without further ado, here is a very simple (and very wrong) metaclass example.

class Z(type):
    def __new__(cls, name, bases, attrs, ):
        print cls, name, bases, attrs

class A(object):
    __metaclass__ = Z


print A
print A()

Running this gives this output.

shabda@shabda-laptop ~/docs> python testmetaclass.py
<class '__main__.Z'> A (<type 'object'>,) {'__module__': '__main__', '__metaclass__': <class '__main__.Z'>}
None
Traceback (most recent call last):
  File "testmetaclass.py", line 14, in <module>
    print A()
TypeError: 'NoneType' object is not callable

Let what happened,

1. print cls, name, bases, attrs got executed, even though we are not creating Z or calling it anywhere.
2. print A printed None
3. print A() failed with a traceback

Lets try a slightly modified file,http://www.agiliq.com/blog/2009/11/the-magic-of-metaclasses-in-python/

class Z(type):
    def __new__(cls, name, bases, attrs, ):
        return str

class A(object):
    __metaclass__ = Z

This gives me,

print A
print A('Hello')

shabda@shabda-laptop ~/docs> python testmetaclass.py
<type 'str'>
Hello
shabda@shabda-laptop ~/docs>

The output suggests that A is behaving like str. This should lead us to a few conclusions,

1. If a class has a __metaclass__, it is replaced by another class.
2. The function called for creating the new class is __new__, it takes 4 parameters.
3. cls is the metaclass being called, name is the name of the original class, bases are bases for original class, and attrs are attributes from original class.
4. The old class is set to the value returned from __metaclass__.__new__
5. As we returned nothing in the first snippet, A was set to None. In second snippet it was set to str.

Here is an actual metaclass being used by Django.

1. In line 24, Form. __metaclass__ is set to DeclarativeFieldsMetaclass
2. DeclarativeFieldsMetaclass is a subclass of type.
3. It defines a method called as __new__ which returns the newly created class.

And that is all there is to metaclasses!

This is the first in the series of short and sweet Django posts we are going to do. You are interested, right. Do follow us on twitter or subscribe to our feed.

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