Metaclasses: Blueprints of Blueprints - The Barebones (
Page 2 of 5 )
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
<class '__main__.A'>
We can see our metaclass of our class like this:
>>> A.__class__
<class '__main__.MetaClass'>
Similarly, we can check the metaclass of an object like so:
>>> z = A()
>>> z.__class__.__class__
<class '__main__.MetaClass'>
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
4
>>> 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.
