Dan Brown wrote: > Why does extending a list with the empty list result in None? It > seems very counterintuitive to me, at least --- I expected ['a'].extend > ([]) to result in ['a'], not None.
How very inconvenient of Python! What it actually does is create an anonymous list containing only the element 'a', and leave it unchanged by extending it with an empty list. Since there is no longer any reference to the list it has become garbage. Contrast that with: >>> lst = ['a'] >>> lst.extend([]) >>> lst ['a'] >>> lst.append([]) >>> lst ['a', []] >>> lst.extend(['1']) >>> lst ['a', [], '1'] >>> As you can see by the absence of output, both the .extend() and .append() list methods return None. They mutate the list instance upon which they are called. In your example you were expecting the methods to return the mutated list. They don't. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 PyCon is coming! Atlanta, Feb 2010 http://us.pycon.org/ Holden Web LLC http://www.holdenweb.com/ UPCOMING EVENTS: http://holdenweb.eventbrite.com/ -- http://mail.python.org/mailman/listinfo/python-list
