The list methods append() and sort() change a list in place but don't return the list. They return None. Don't use these methods in assignment statements or else the list might disappear, as in this example:
>>> L = ["my", "list"]
To append to a list, just use the append() method by itself. Don't give the result a name.
>>> L = ["my", "list"] >>> L.append('x') >>> print L ['my', 'list', 'x']
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