Python Cookbook

Implementing Class Methods

Credit: Thomas Heller

Problem

You want to call methods directly on a class without having to supply an instance, and with the class itself as the implied first argument.

Solution

In Python 2.2 (on either classic or new-style classes), the new built-in classmethod function wraps any callable into a class method, and we just bind the same name to the classmethod object in class scope:

class Greeter:
    def greet(cls, name): print "Hello from %s"%cls._ _name_ _, name
    greet = classmethod(greet)

In Python 2.1 or earlier, we need a wrapper that is slightly richer than the one used for static methods in Recipe 5.7:

class classmethod:
    def _ _init_ _(self, func, klass=None):
        self.func = func
        self.klass = klass
    def _ _call_ _(self, *args, **kw):
        return self.func(self.klass, *args, **kw)

Furthermore, with this solution, the following rebinding is not sufficient:

greet = classmethod(greet)

This leaves greet.klass set to None, and if the class inherited any class methods from its bases, their klass attributes would also be set incorrectly. It’s possible to fix this by defining a function to finish preparing a class object and always explicitly calling it right after every class statement. For example:

def arrangeclassmethods(cls):
    for attribute_name in dir(cls):
        attribute_value = getattr(cls, attribute_name)
        if not isinstance(attribute_value, classmethod): continue
        setattr(cls, classmethod(attribute_value.func, cls))

However, this isn’t completely sufficient in Python versions ...