intercept.py

class A(object):
    fmt = "tweets %s"
    def hello(self):
        return "hello " + self.name

    @classmethod
    def tweets(cls, mes):
        return cls.fmt % mes
    def __init__(self, name):
        self.name = name

class B(object):
    fmt = "called with B: %s"
    def __init__(self, name):
        self.name = name

def intercept(cls, method):
    def doaction(caller, *args, **kwargs):
        caller.__class__ = cls
        return method.im_func(caller, *args, **kwargs)
    return doaction


print A("foo").hello()
try:
    print A.hello(B("bar"))
except TypeError, e:
    print e
print intercept(A, A.hello)(B("bar"))

print "=="

print A.tweets("hey")
B.tweets = A.__dict__["tweets"]
print B.tweets("bar")