Initialization of member variables with lists leads to strange behavior. The list object is common to each instance of that class. File attached results in: [] [] <__main__.Proof instance at 0x00BA7120> ['STICKYARRAY'] [] <__main__.Proof instance at 0x00BA7148> So the initialized list a is the same in both instances. Behaviour has been tested on 2.3 and 2.5