Defining a list ``[]`` or dictionary ``{}`` as a default parameter this way doesn't work as expected. This default list will only be created the first time this file is loaded by the interpreter and then used every time this function is called without the ``into`` parameter. I think [this article on stackoverflow](http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument) explains it quite well...better than I probably could anyway :)