sorting a python list by two criteria

Sorting a Python list by two criteria

up vote 59 down vote favorite 20	I have the following list created from a sorted csv list1 = sorted(csv1, key=operator.itemgetter(1)) I would actually like to sort the list by two criteria: first by the value in field 1 and then by the value in field 2. How do I do this? python  sorting share improve this question asked  Mar 6 '11 at 19:36 half full 402 1 5 8
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up vote 66 down vote accepted

like this: import operator list1 = sorted(csv1, key=operator.itemgetter(1, 2)) share improve this answer edited Sep 4 '15 at 8:54 BDM 332 1 3 14 answered  Mar 6 '11 at 19:38 mouad 31.2k 6 67 83

1   +1: More elegant than mine. I forgot that itemgetter can take multiple indices. –  dappawit  Mar 6 '11 at 19:43 @half full: glad it help :) –  mouad  Mar 6 '11 at 19:53 6   operator is a module that needs to be imported. –  trapicki  Aug 28 '13 at 14:45  how will i proceed if i want to sort ascending on one element and descending on other, using itemgetter??. –  ashish  Oct 12 '13 at 10:13  1   @ashish, see my answer below with lambda functions this is clear, sort by "-x[1]" or even "x[0]+x[1]" if you wish –  jaap  Feb 27 '14 at 15:15 show 1 more comment

up vote 100 down vote

Replying to this dead thread for archive. No need to import anything when using lambda functions. The following sorts list by the first element, then by the second element. sorted(list, key=lambda x: (x[0], -x[1])) share improve this answer edited Jan 14 at 20:39 answered  Jun 14 '13 at 13:01 jaap 1,446 1 9 17

I like this solution because you can convert strings to int for sorting like: lambda x: (x[0],int(x[1])). +1 –  pbible  Sep 10 '14 at 13:41  4   Nice. As you noted in comment to the main answer above, this is the best (only?) way to do multiple sorts with different sort orders. Perhaps highlight that. Also, your text does not indicate that you sorted descending on second element. –  PeterVermont  Jun 12 '15 at 14:25 Also what if  x[1] is date? Should I convert it to integer also? @pbible does conversion of string to int preserve the alphabetical ordering of the string? –  user1700890  Nov 20 '15 at 23:32  1   @user1700890 I was assuming the field was already string. It should sort strings in alphabetical order by default. You should post your own question separately on SO if it is not specifically related to the answer here or the OP's original question. –  pbible  Nov 23 '15 at 16:59 2   @jan it's reverse sort –  jaap  Feb 28 at 23:39 show 7 more comments

up vote 8 down vote

Python has a stable sort, so provided that performance isn't an issue the simplest way is to sort it by field 2 and then sort it again by field 1. That will give you the result you want, the only catch is that if it is a big list (or you want to sort it often) calling sort twice might be an unacceptable overhead. list1 = sorted(csv1, key=operator.itemgetter(2)) list1 = sorted(list1, key=operator.itemgetter(1)) Doing it this way also makes it easy to handle the situation where you want some of the columns reverse sorted, just include the 'reverse=True' parameter when necessary. Otherwise you can pass multiple parameters to itemgetter or manually build a tuple. That is probably going to be faster, but has the problem that it doesn't generalise well if some of the columns want to be reverse sorted (numeric columns can still be reversed by negating them but that stops the sort being stable). So if you don't need any columns reverse sorted, go for multiple arguments to itemgetter, if you might, and the columns aren't numeric or you want to keep the sort stable go for multiple consecutive sorts. Edit: For the commenters who have problems understanding how this answers the original question, here is an example that shows exactly how the stable nature of the sorting ensures we can do separate sorts on each key and end up with data sorted on multiple criteria: DATA = [ ('Jones', 'Jane', 58), ('Smith', 'Anne', 30), ('Jones', 'Fred', 30), ('Smith', 'John', 60), ('Smith', 'Fred', 30), ('Jones', 'Anne', 30), ('Smith', 'Jane', 58), ('Smith', 'Twin2', 3), ('Jones', 'John', 60), ('Smith', 'Twin1', 3), ('Jones', 'Twin1', 3), ('Jones', 'Twin2', 3) ] # Sort by Surname, Age DESCENDING, Firstname print("Initial data in random order") for d in DATA: print("{:10s} {:10s} {}".format(*d)) print(''' First we sort by first name, after this pass all Twin1 come before Twin2 and Anne comes before Fred''') DATA.sort(key=lambda row: row[1]) for d in DATA: print("{:10s} {:10s} {}".format(*d)) print(''' Second pass: sort by age in descending order. Note that after this pass rows are sorted by age but Twin1/Twin2 and Anne/Fred pairs are still in correct firstname order.''') DATA.sort(key=lambda row: row[2], reverse=True) for d in DATA: print("{:10s} {:10s} {}".format(*d)) print(''' Final pass sorts the Jones from the Smiths. Within each family members are sorted by age but equal age members are sorted by first name. ''') DATA.sort(key=lambda row: row[0]) for d in DATA: print("{:10s} {:10s} {}".format(*d)) This is a runnable example, but to save people running it the output is: Initial data in random order Jones Jane 58 Smith Anne 30 Jones Fred 30 Smith John 60 Smith Fred 30 Jones Anne 30 Smith Jane 58 Smith Twin2 3 Jones John 60 Smith Twin1 3 Jones Twin1 3 Jones Twin2 3 First we sort by first name, after this pass all Twin1 come before Twin2 and Anne comes before Fred Smith Anne 30 Jones Anne 30 Jones Fred 30 Smith Fred 30 Jones Jane 58 Smith Jane 58 Smith John 60 Jones John 60 Smith Twin1 3 Jones Twin1 3 Smith Twin2 3 Jones Twin2 3 Second pass: sort by age in descending order. Note that after this pass rows are sorted by age but Twin1/Twin2 and Anne/Fred pairs are still in correct firstname order. Smith John 60 Jones John 60 Jones Jane 58 Smith Jane 58 Smith Anne 30 Jones Anne 30 Jones Fred 30 Smith Fred 30 Smith Twin1 3 Jones Twin1 3 Smith Twin2 3 Jones Twin2 3 Final pass sorts the Jones from the Smiths. Within each family members are sorted by age but equal age members are sorted by first name. Jones John 60 Jones Jane 58 Jones Anne 30 Jones Fred 30 Jones Twin1 3 Jones Twin2 3 Smith John 60 Smith Jane 58 Smith Anne 30 Smith Fred 30 Smith Twin1 3 Smith Twin2 3 Note in particular how in the second step the reverse=True parameter keeps the firstnames in order whereas simply sorting then reversing the list would lose the desired order for the third sort key. share improve this answer edited Jan 22 '15 at 9:44 answered  Mar 6 '11 at 19:49 Duncan 38.6k 6 43 94

Thanks that's very helpful. –  half full  Mar 6 '11 at 19:53 Stable sorting doesn't mean that it won't forget what your previous sorting was. This answer is wrong. –  Mike Axiak  Mar 6 '11 at 21:10 1   Stable sorting means that you can sort by columns a, b, c simply by sorting by column c then b then a. Unless you care to expand on your comment I think it is you that is mistaken. –  Duncan  Mar 6 '11 at 21:23  2   This answer is definitely correct, although for larger lists it's unideal: if the list was already partially sorted, then you'll lose most of the optimization of Python's sorting by shuffling the list around a lot more. @Mike, you're incorrect; I suggest actually testing answers before declaring them wrong. –  Glenn Maynard  Mar 6 '11 at 21:39 2   @MikeAxiak: docs.python.org/2/library/stdtypes.html#index-29 states in comment 9: Starting with Python 2.3, the sort() method is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal — this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade). –  trapicki  Aug 28 '13 at 14:40  show 5 more comments

up vote 3 down vote	def keyfunc(x): return tuple(x[1],x[2]) list1 = sorted(csv1, key=keyfunc) share improve this answer answered  Mar 6 '11 at 19:41 dappawit 5,077 1 15 21
	1   I don't think tuple() can receive two arguments (or rather, three, if you count with self) –  Filipe Correia Dec 12 '12 at 23:15 1   tuple takes only can take one argument –  therealprashant  Jun 6 '15 at 11:02