Now that's a splendid example. The previous Guy-Amit discussion was on a somewhat higher level (not actually relating to the specific ongoing course), but surely when teaching the kids it's better to use this kind of examples.
R. -----Original Message----- From: Nir Soffer [mailto:[EMAIL PROTECTED] Sent: Sunday, December 04, 2005 3:58 AM To: RaeNye Cc: Python User Group Subject: Re: teaching python variables On 4 Dec, 2005, at 2:09, RaeNye wrote: >> Why use this strange name "mlist_a"? Instead use: >> multiple_lists = [list1, list2] > > I don't see a problem with Guy's name selection. I consider your > suggestion worse, as the object called "multiple_lists" is, in fact, a > single list that contains two other lists. You might call the outer > list "nested_list" > or > "outer_list", but "multiple_lists" has nothing to do with the object > itself. "lists" will be a better name. But why the names in this code are so lousy? because its not a concrete example, its too abstract. Using real life examples can make everything more clear. For example, bart simpson downloaded our first game, and lisa created a list of scores for each family member: >>> lisa_scores = [90, 85, 80] >>> bart_scores = [50, 65] >>> homer_scores = [] # too stupid to play >>> marge_scores = [] # too busy # Lets keep all the scores in one list >>> simpsons_scores[lisa_scores, bart_scores, homer_scores, marge_scores] Now we can ask questions about it: What were lisa scores? >>> simpsons_scores[0] [90, 85, 80] >>> lisa_scores [90, 85, 80] they look equal... >>> simpsons_scores[0] == lisa_scores True They are. Is it the same list of a copy? >>> simpsons_scores[0] is lisa_scores True This answer the question, what happens when you do list = [foo, bar] - you create reference to the objects. homer and marge have both empty list. They must be equal: >>> homer_scores == marge_scores True But they are not the same list: >>> homer_scores is marge_scores False lisa teach homer to play, and add some scores: >>> homer_scores.append(10) the simpsons_scores list updated: >>> simpsons_scores [[90, 85, 80], [50, 65], [10], []] homer_scores and simpsons_scores[2] reference the same list object (which Python manage for us somewhere in the memory) homer tries again, made some improvement: >>> homer_scores.append(11) >>> simpsons_scores [[90, 85, 80], [50, 65], [10, 11], []] bart is not happy with lisa results: >>> del lisa_scores >>> lisa_scores Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'lisa_scores' is not defined That was evil! >>> simpsons_scores [[90, 85, 80], [50, 65], [10, 11], []] F**k! they are still there!? hmm, lisa_scores was not the list, its only a name, the list is still there. This is exactly what Python told us just now: "NameError: name 'lisa_scores' is not defined". lisa fixes the situation >>> lisa_scores = simpsons_scores[0] >>> lisa_scores [90, 85, 80] Now [[90, 85, 80], [50, 65], [10, 11], []] is a lousy way to keep this kind of data, because we have to remember the order of the family members. This leads naturally to dicts - because we need it NOW to write simpler and more readable code. I hope it helps, It was fun to write :-) Best Regards, Nir Soffer
