gistfile1.py

import inspect

class Y(object):
    def ymethod(self, x):
        x.method()

class X(object):
    def method(self):
        frame = inspect.currentframe()
        prev = frame.f_back
        caller = prev.f_locals.get('self') # hope everyone calls it 'self'
        assert type(caller) == Y
        print caller

Y().ymethod(X())