# Python Programming problem - Lists and += **URL:** \ **Category:** Factual Questions\ **Created:** [February 11, 2006, 5:24pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006 "2006-02-11T17:24:47Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![silverfish](https://avatars.discourse-cdn.com/v4/letter/s/e480ec/32.png) [@silverfish](https://boards.straightdope.com/u/silverfish)\ **Post date:** [February 11, 2006, 5:24pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/1 "2006-02-11T17:24:47Z")
I’m learning about Python (version 2.4.2 if that makes any differences), but I’ve come across something that doesn’t seem to do what I want it to do. I gather that a += b, for example, is the equivalent of a = a + b. This seems to work for numbers, thus: \>\>\> a = 3 \>\>\> a += 2 \>\>\> print a \>\>\> 5 However, it seems to do odd things when you have lists of lists. For example: \>\>\> a = [[9]]\*10 \>\>\> print a [[9], [9], [9], [9], [9], [9], [9], [9], [9], [9]] \>\>\> a[5] += [3] \>\>\> print a [[9, 3], [9, 3], [9, 3], [9, 3], [9, 3], [9, 3], [9, 3], [9, 3], [9, 3], [9, 3]] \>\>\> What I would expect a[5] += [3] to do is append 3 to the 6th entry in a, to make a[5] = [9.3], and the other entries equal to [9]. However, it appears here to append it to all the lists in a. Is this just a quirk in Python, or something I’m doing wrong? Can I use += in this way to do what I want, and/or is there another (better) way of doing it? To clarify, what I want to do sometimes involves appending more than one element to lists, such as appending 3 and 4, to get [9,3,4]. ---
**Author:** ![alterego](https://avatars.discourse-cdn.com/v4/letter/a/6bbea6/32.png) [@alterego](https://boards.straightdope.com/u/alterego)\ **Post date:** [February 11, 2006, 5:47pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/2 "2006-02-11T17:47:26Z")
I happened to have #python open on freenode, so I passed your question along. Hope this helps: ```auto me: how does the += operator work on lists? why does my second statement here add an indice with the number three to each field? me: a = [[9]]*10;a[5] += [3];print a notme: += is the .extend method. notme: but, [[9]] * 10 will create a list with 10 references to the same sublist ( [9] ) notme: __iadd__ = extend ... notme: when you do [[9]] * 10, you're repeating the same reference, it's the same list notme: use something like [[9] for i in xrange(10)] ``` ---
**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\ **Post date:** [February 11, 2006, 5:53pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/3 "2006-02-11T17:53:58Z")
**a = [_x_] \* 9** creates a list with 10 _references_ to _x_. And **a[_y_] = _z_** modifies whatever a[_y_] points to (not a[_y_] itself), which would be the same thing that all the other elements of list a point to. ---
**Author:** ![silverfish](https://avatars.discourse-cdn.com/v4/letter/s/e480ec/32.png) [@silverfish](https://boards.straightdope.com/u/silverfish)\ **Post date:** [February 11, 2006, 6:52pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/4 "2006-02-11T18:52:07Z")
Thanks for the help. I think I get what’s happening. I’ve had a bit more of a look, and other methods applied to a[5] are the same. (e.g. a[5].sort() sorts all members of a, or rather sorts the one list they are all pointing to, and similar applies to extend(), which I gather from alterego is what += actually uses for lists. Will all list methods treat [[9]]\*10 (and smilar), in this way? Also, a[5] = a[5] + [2] only affects a[5] as I would expect. Do I take it if I use a[5] = a[5] + x, then that creates a seperate “slot”, for a[5] to point to, seperate from the other members of a? In any case, the a = a + x method seems to do what I want it to do in the program I’m writing at the moment, so I’ll probably probably leave it at that for this program. ---
**Author:** ![silverfish](https://avatars.discourse-cdn.com/v4/letter/s/e480ec/32.png) [@silverfish](https://boards.straightdope.com/u/silverfish)\ **Post date:** [February 11, 2006, 7:42pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/5 "2006-02-11T19:42:50Z")
Thanks for the help. I think I get what’s happening. I’ve had a bit more of a look, and other methods applied to a[5] are the same. (e.g. a[5].sort() sorts all members of a, or rather sorts the one list they are all pointing to, and similar applies to extend(), which I gather from alterego is what += actually uses for lists. Will all list methods treat [[9]]\*10 (and smilar), in this way? Also, a[5] = a[5] + [2] only affects a[5] as I would expect. Do I take it if I use a[5] = a[5] + x, then that creates a seperate “slot”, for a[5] to point to, seperate from the other members of a? In any case, the a = a + x method seems to do what I want it to do in the program I’m writing at the moment, so I’ll probably probably leave it at that for this program. ---
**Author:** ![Ximenean](https://avatars.discourse-cdn.com/v4/letter/x/aca169/32.png) [@Ximenean](https://boards.straightdope.com/u/Ximenean)\ **Post date:** [February 12, 2006, 11:57am UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/6 "2006-02-12T11:57:17Z")
It’s not so much a quirk of lists, it’s more that there is a fundamental difference between the assignment operator ‘=’ and “augmented” operators like ‘+=’. The ‘=’ operator always creates a new reference, while ‘+=’ never does. So in your example, **a[5] = a[5] + [2]** makes a[5] point to an entirely new list of the form [[9], [2]], whereas **a[5] += [2]** doesn’t change the reference a[5] itself, it changes whatever it points to (in this case, the list [9] that all the other members of _a_ happen to point to). ---
**Author:** ![silverfish](https://avatars.discourse-cdn.com/v4/letter/s/e480ec/32.png) [@silverfish](https://boards.straightdope.com/u/silverfish)\ **Post date:** [February 12, 2006, 1:44pm UTC](https://boards.straightdope.com/t/python-programming-problem-lists-and/344006/7 "2006-02-12T13:44:34Z")
I think I understand now. += acts in a similar way for other data types (integers, strings, etc), as for lists, it’s just that it’s only when you create multiple elements pointing to the same thing, as with [[9]]\*10, that you actually notice it. What about other methods, such as x.sort()? Do they act on whatever x points to, rather than create a new reference (as a = a +b does)?