gistfile1.py

#!/usr/bin/python

class A(object):
    field = 0

    def __hash__(self):
        return hash(self.field)

    def __eq__(self, other):
        return self.field == other.field

a = A()
myset = set()
mydict = {}

myset.add(a)
mydict[a] = 1
assert a in myset

a.field = 100
assert a not in myset
assert a not in mydict
assert mydict.items() == [(a, 1)]