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Built-in Decorators

Python has two built-in decorators.

@staticmethod

The staticmethod decorator modifies a method function so that it does not use the self variable. The method function will not have access to a specific instance of the class.

For an example of a static method, see the section called “Static Methods and Class Method”.

@classmethod

The classmethod decorator modifies a method function so that it receives the class object as the first parameter instead of an instance of the class. This method function wil have access to the class object itself.

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 )
1

We import the types module to help us distinguish among the various elements of a class definition.

2

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.

3

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.

4

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')]

 
 
  Published under the terms of the Open Publication License Design by Interspire