Python has two built-in decorators.

The @classmethod decorator is used to create singleton classes. This is a Python technique for defining an object which is also a unique class. The class definition is also the one and only instance. This gives us a very handy, easy-to-read way to segregate attributes into a separate part of a class declaration. This is a technique used heavily by Python frameworks.

Generally, a function decorated with @classmethod is used for introspection of a class. An introspection method looks at the structure or features of the class, not the values of the specific instance.

Here's a contrived example of using introspection to display some features of a object's class.

Example 26.1. introspection.py

import types

class SelfDocumenting( object ):
    @classmethod
    def getMethods( aClass ):
        return [ (n,v.__doc__) for n,v in aClass.__dict__.items()
                 if type(v) == types.FunctionType ]
    def help( self ):
        """Part of the self-documenting framework"""
        print self.getMethods()

class SomeClass( SelfDocumenting ):
    attr= "Some class Value"
    def __init__( self ):
        """Create a new Instance"""
        self.instVar= "some instance value"
    def __str__( self ):
        """Display an instance"""
        return "%s %s" % ( self.attr, self.instVar )

	We import the types module to help us distinguish among the various elements of a class definition.
	We define a superclass that includes two methods. The classmethod, getMethods, introspects a class, looking for the method functions. The ordinary instance method, help, uses the introspection to print a list of functions defined by a class.
	We use the @classmethod decorator to modify the getMethods function. Making the getMethods into a class method means that the first argument will be the class object itself, not an instance.
	Every subclass of SelfDocumenting can print a list of method functions using a help method.

Here's an example of creating a class and calling the help method we defined. The result of the getMethods method function is a list of tuples with method function names and docstrings.

>>> 
ac= SomeClass()

>>> 
ac.help()

[('__str__', 'Display an instance'), ('__init__', 'Create a new Instance')]