Calling a class method upon creation of Python classes

Question

I'd like to automatically run some code upon class creation that can call other class methods. I have not found a way of doing so from within the class declaration itself and end up creating a @classmethod called __clsinit__ and call it from the defining scope immediately after the class declaration. Is there a method I can define such that it will get automatically called after the class object is created?

Answer 1

You can do this with a metaclass or a class decorator.

A class decorator (since 2.6) is probably easier to understand:

def call_clsinit(cls):
    cls._clsinit()
    return cls

@call_clsinit
class MyClass:
    @classmethod
    def _clsinit(cls):
        print "MyClass._clsinit()"

Metaclasses are more powerful; they can call code and modify the ingredients of the class before it is created as well as afterwards (also, they can be inherited):

def call_clsinit(*args, **kwargs):
    cls = type(*args, **kwargs)
    cls._clsinit()
    return cls;

class MyClass(object):
    __metaclass__ = call_clsinit

    @classmethod
    def _clsinit(cls):
        print "MyClass._clsinit()"