Re: [Tutor] For loop breaking string methods
On 04/27/10 12:19, Dave Angel wrote: Note also that if you insert or delete from the list while you're looping, you can get undefined results. That's one reason it's common to build a new loop, and just assign it back when done. Example would be the list comprehension you showed earlier. I have to add to Dave's statement, if you modify the list's content while looping there is no undefined behavior; you get undefined behavior if you modify the list's structure. Operations that modify a list's structure includes insertion, delete, append, etc; those operations which can modify the len() of list. Modifying the content, however, is perfectly safe. However, even when just modifying list's content, I personally still prefer list/generator comprehension. They're fast and simple. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
On 4/26/2010 10:19 PM, Dave Angel wrote: Note also that if you insert or delete from the list while you're looping, you can get undefined results. The results are not undefined. They m,ight be unexpected, but that is due to an invalid assumption. The behavior is explained in section 7.3 of The Python Language Reference. Note especially: There is a subtlety when the sequence is being modified by the loop (this can only occur for mutable sequences, i.e. lists). An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. When this counter has reached the length of the sequence the loop terminates. This means that if the suite deletes the current (or a previous) item from the sequence, the next item will be skipped (since it gets the index of the current item which has already been treated). Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g., for x in a[:]: if x0: a.remove(x) -- Bob Gailer 919-636-4239 Chapel Hill NC ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
On 4/26/2010 3:38 PM, C M Caine wrote: Why does this not work: By work you mean do what I want it to do. L = [' foo ','bar '] for i in L: i = i.strip() This creates a new local variable named i. It does not affect L. This has nothing to do with loops nor is it strange behavior - it is expected behavior. Consider the loopless equivalent: i = L[0] i = i.strip() L [' foo ', 'bar '] L [' foo ', 'bar '] # note the leading whitespace that has not been removed. But this does: L = [i.strip() for i in L] L ['foo', 'bar'] What other strange behaviour should I expect from for loops? None - loops do not have strange behaviors. Perhaps unexpected but that is a result of not understanding an aspect of the language. -- Bob Gailer 919-636-4239 Chapel Hill NC ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
Thank you for the clarification, bob. For any future readers of this thread I include this link[1] to effbot's guide on lists, which I probably should have already read. My intention now is to modify list contents in the following fashion: for index, value in enumerate(L): L[0] = some_func(value) Is this the standard method? [1]: http://effbot.org/zone/python-list.htm Colin Caine ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
On 26 April 2010 21:38, C M Caine cmca...@googlemail.com wrote: Why does this not work: L = [' foo ','bar '] for i in L: i = i.strip() str.strip() _returns_ a *new* string and leaves the original string alone. The reason being that string are immutable so can not be changed. s1 = ' foo ' s1[1] 'f' s1[1] = 'g' Traceback (most recent call last): File pyshell#7, line 1, in module s1[1] = 'g' TypeError: 'str' object does not support item assignment However lists are mutable and can be changed in place. l = [' foo ','bar '] l[0] = ' boo ' l [' boo ', 'bar '] l[1] = 'far ' l [' boo ', 'far '] But this does: L = [i.strip() for i in L] L ['foo', 'bar'] What you do here is create a *new* list object and assign it to variable L. The new list is created with *new* string objects returned by str.strip(). Putting the 2 together you could do something like below but I would use a list comprehension like you did above: l = [' foo ','bar '] for x in range(len(l)): l[x] = l[x].strip() l ['foo', 'bar'] What other strange behaviour should I expect from for loops? You should read up on immutable data types like strings and tuples. Start with [1]. Greets Sander [1] http://docs.python.org/reference/datamodel.html ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
C M Caine cmca...@googlemail.com wrote My intention now is to modify list contents in the following fashion: for index, value in enumerate(L): L[0] = some_func(value) I think you mean: L[index] = some_func(value) Is this the standard method? Or use a List copmprehension. L = [some_func(value) for value in L] I'd say nowadays that the comprehension was most common. -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
What other strange behaviour should I expect from for loops? You should read up on immutable data types like strings and tuples. Start with [1]. Greets Sander [1] http://docs.python.org/reference/datamodel.html Thank you kindly for your reply, I'll be sure to read up on it. Colin ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
On 26 April 2010 23:45, Alan Gauld alan.ga...@btinternet.com wrote: C M Caine cmca...@googlemail.com wrote My intention now is to modify list contents in the following fashion: for index, value in enumerate(L): L[0] = some_func(value) I think you mean: L[index] = some_func(value) Yes, I do Is this the standard method? Or use a List copmprehension. L = [some_func(value) for value in L] I'd say nowadays that the comprehension was most common. -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ Thanks, I can see why the comprehensions are more popular. Colin ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] For loop breaking string methods
C M Caine wrote: Thank you for the clarification, bob. For any future readers of this thread I include this link[1] to effbot's guide on lists, which I probably should have already read. My intention now is to modify list contents in the following fashion: for index, value in enumerate(L): L[0] = some_func(value) Is this the standard method? [1]: http://effbot.org/zone/python-list.htm Colin Caine Almost. You should have said L[index] = some_func(value) The way you had it, it would only replace the zeroth item of the list. Note also that if you insert or delete from the list while you're looping, you can get undefined results. That's one reason it's common to build a new loop, and just assign it back when done. Example would be the list comprehension you showed earlier. DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
