Here[0]. Mutable objects as default values create a single instance of the object. For iterables (list, set, etc) the behaviour is particularly surprising coz it accumulates/memoize every item ever added (i.e a list that grows/leaks with every call to the function). If you use a good IDE (like pycharm), it will warn you about this stuff.

[0] http://docs.python-guide.org/en/latest/writing/gotchas/