It's an obscure Python quirk that default parameters are constructed during compilation time and then never re-constructed. So, to illustrate: >>> def hello(

It's an obscure Python quirk that default parameters are constructed during compilation time and then never re-constructed. So, to illustrate: >>> def hello(a=[]): a.append('hello'); return a; ... >>> hello() ['hello'] >>> hello() ['hello', 'hello'] On post, what mindslight said.