switching.py

class SwitchingMethod(object):
    """This decorator turns a method into a classmethod, 
    but only when it's being called from the class; 
    it still works perfectly from the instance."""
    def __init__(self, f):
        self.f = f
    def __get__(self, obj, klass=None):
        if obj is None:
            return classmethod(self.f).__get__(obj, klass)
        return self.f.__get__(obj, klass)

class Test(object):
    @SwitchingMethod
    def hello(self_or_cls, msg):
        print "Hello! %s" % msg

Test.hello("I am being called on the class!")
Test().hello("I am being called on an instance!")