example.py

import gc
gc.set_debug(gc.DEBUG_UNCOLLECTABLE | gc.DEBUG_STATS)

class CallbackTest (object):
    def __init__(self):
        self.my_callback = lambda :None

    def signal(self):
        self.my_callback()

class App (object):

    def __init__(self):
        self.test = CallbackTest()
        self.test.my_callback = self.on_signal
        self.test.signal()

    def on_signal(self):
        print "Hello!"

App()

output.txt

Hello!
gc: collecting generation 2...
gc: objects in each generation: 97 3117 0
gc: done, 5 unreachable, 0 uncollectable, 0.0015s elapsed.