import collections


class keydefaultdict(collections.defaultdict):
    """Improved defaultdict to pass the requested key to factory function."""
    def __missing__(self, key):
        print 'accessed keydefaultdict.__missing__(%s)' % key
        if self.default_factory is None:
            raise KeyError("Invalid key '{0}' and no default factory was set")
        else:
            val = self.default_factory(key)
            self[key] = val
            return val


def foo(k):
    return k + k

kdd = keydefaultdict(foo)
# line below causes KeyError in Jython, ignoring overridden __missing__ method
assert kdd[3] == 6
assert kdd['ab'] == 'abab'