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Safari | Python Developer's Handbook -> Metaclasses





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Metaclasses

A metaclass is just a class that is used as a template to create class-like entities.

Normally, you create instances based on classes. The goal here is to create classes (metainstances) based on other classes (metaclasses). The resulting metainstances are used as base classes for your own classes.

The whole idea is to offer you the possibility of operating Python's internal class-handling engine. Everything that usually happens behind the scenes while manipulating your classes and objects now can be accessed and changed. The meta instance makes it easier for you to handle the task of modifying the attribute lookup behavior of objects.

Prior to Python, version 1.5, it was necessary to use C extensions in order to define metaclasses.

The subsequent code defines a simple metaclass and its supporting classes. Note that this structure doesn't cover the whole model.

					
 1: >>> import types
 2: >>> class METACLASS:
 3:     def __init__(self, name, bases, namespace):
 4:         self.__name__ = name
 5:         self.__bases__ = bases
 6:         self.__namespace__ = namespace
 7:     def __call__(self):
 8:         return METAINSTANCE(self)
 9: 
10: >>> class METAINSTANCE:
11: def __init__(self, metaclass):
12:     self.__metaclass__ = metaclass
13: def __getattr__(self, name):
14:     try:
15:         value = self.__metaclass__.__namespace__[name]
16:     except KeyError:
17:         raise AttributeError, name
18:     if type(value) is not types.FunctionType:
19:         return value
20:     return METHODWRAPPER(value, self)
21: 
22: >>> class METHODWRAPPER:
23: def __init__(self, function, metainstance):
24:     self.function = function
25:     self.instance = metainstance
26:     self.__name__ = self.function.__name__
27: def __call__(self, *args):
28:     return apply(self.function, (self.instance,) + args)
29: 

				

Line 2 : Defines the metaclass METACLASS.

Lines 3-6 : Creates a new metaclass. The __init__ method expects three arguments: The metainstance name, a tuple of base classes, and a dictionary of the metainstance namespace.

Lines 7-8 : Invokes METAINSTANCE.__init__ when METACLASS is called, returning a metainstance.

Line 10 : Defines the metainstance METAINSTANCE.

Line 13 : Handles the access to attributes of the user instance by checking whether it is part of the user class namespace (lines 14-17). If the attribute is a value, it returns the value. Otherwise, if the attribute is a function, it returns an instance of the METHODWRAPPER class, which is actually the result of the function call.

Line 22 : Defines the METHODWRAPPER class, which handles all the accesses to the method attributes of the user class.

Now that we are ready to call metaclasses, you can use metainstances as base classes of your own classes, trapping the access to your class objects. The next line of code creates an instance of a metainstance.

					
>>> BASECLASS = METACLASS('BASECLASS', (), {
 
} )

				

Let me explain to you what is really happening here:

We are creating a class called BASECLASS whose behavior is inherited from the METACLASS constructor class. The METACLASS.__init__ method is invoked at this stage.

From now on, every class that you createwhich uses BASECLASS as the base classwill inherit the whole behavior that you have specified in the METACLASS definition.

The following code exemplifies a user class that has our BASECLASS as the base class.

					
>>> class CEO(BASECLASS):
    def push(self, name):
        self.name = [name]
    def pop(self):
        if len(self.name) > 0:
            item = self.name[-1]
            del self.name[-1]
            print item


				

Now it's time to illustrate the use of this whole concept.

					
>>> ITCEO = CEO()
>>> ITCEO.push("Andre")
>>> ITCEO.pop()
['Andre']
>>> ITCEO.name
[]

				

Note that ITCEO = CEO() invokes METACLASS.__call__, which creates a METAINSTANCE instance, whereas all the other calls invoke METAINSTANCE.__getattr__.

More details about metaclasses can be found at the following addresses:

http://www.python.org/doc/essays/metaclasses/

and MessThe Meta-Extension System Set (old stuff) at http://starship.python.net/crew/da/mess/doc/Tutorial.

Mess is a set of extensions that allows the creation of new types, among other things. It's not certain whether it will ever be integrated into Python, but its documentation can provide a lot of help in understanding metaclass concepts.

Maybe you will like to take a look at the ExtensionClass extension by Digital Creations that uses metaclasses to allow creation of class-like objects in C (and is a lot easier to use than Mess). This extension illustrates how the Python class mechanism can be extended, and provides a lightweight mechanism developed for making Python extension types more class-like. Classes can be developed in an extension language, such as C or C++, and treated like other Python classes.

http://www.digicool.com/releases/ExtensionClass/


Last updated on 1/30/2002
Python Developer's Handbook, © 2002 Sams Publishing

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