"kpp9c" <kp8 at mac.com> writes: > self._all_states |= set(key[i] for key in probabilities) > I am running: > Python 2.3 (#1, Sep 13 2003, 00:49:11) generator comprehensions are new in 2.4. Try: self._all_states |= set([key[i] for key in probabilities]) This does temporarily use more memory.