Please read the docs here: http://docs.python.org/py3k/reference/expressions#boolean-operations

This is the last sentence in the '5.10. Boolean operations' text):
([...] Because not has to >>>invent<<< a value anyway, it does not bother to return a value of the same type as its argument, so e.g., not 'foo' yields False, not ''.)

Fix to:
([...] Because not has to >>>invert<<< a value anyway, it does not bother to return a value of the same type as its argument, so e.g., not 'foo' yields False, not ''.)