Sort a list by multiple attributes

Question

I have a list of lists     12   tall    blue   1    2   short    red   9    4   tall    blue   13     If I wanted to sort by one element  say the tall short element  I could do it via s   sorted s  key   itemgetter 1     If I wanted to sort by both tall short and colour  I could do the sort twice  once for each element  but is there a quicker way

User · Answer

There is a operator  lt  between lists e g    12   tall    blue   1   lt   4   tall    blue   13   will give False

User · Answer

It appears you could use a list instead of a tuple   This becomes more important I think when you are grabbing attributes instead of  magic indexes  of a list tuple     In my case I wanted to sort by multiple attributes of a class  where the incoming keys were strings  I needed different sorting in different places  and I wanted a common default sort for the parent class that clients were interacting with  only having to override the  sorting keys  when I really  needed to   but also in a way that I could store them as lists that the class could share  So first I defined a helper method  def attr sort self  attrs   someAttributeString         helper to sort by the attributes named by strings of attrs in order      return lambda k    getattr k  attr  for attr in attrs     then to use it    would defined elsewhere but showing here for consiseness self SortListA     attrA    attrB   self SortListB     attrC    attrA   records         list of my objects to sort records sort key self attr sort attrs self SortListA     perhaps later nearby or in another function more records         another list more records sort key self attr sort attrs self SortListB     This will use the generated lambda function sort the list by object attrA and then object attrB assuming object has a getter corresponding to the string names provided   And the second case would sort by object attrC then object attrA      This also allows you to potentially expose outward sorting choices to be shared alike by a consumer  a unit test  or for them to perhaps tell you how they want sorting done for some operation in your api by only have to give you a list and not coupling them to your back end implementation

User · Answer

Several years late to the party but I want to both sort on 2 criteria and use reverse True  In case someone else wants to know how  you can wrap your criteria  functions  in parenthesis   s   sorted my list  key lambda i    criteria 1 i   criteria 2 i     reverse True

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Here s one way  You basically re-write your sort function to take a list of sort functions  each sort function compares the attributes you want to test  on each sort test  you look and see if the cmp function returns a non-zero return if so break and send the return value   You call it by calling a Lambda of a function of a list of Lambdas   Its advantage is that it does single pass through the data not a sort of a previous sort as other methods do  Another thing is that it sorts in place  whereas sorted seems to make a copy   I used it to write a rank function  that ranks a list of classes where each object is in a group and has a score function  but you can add any list of attributes   Note the un-lambda-like  though hackish use of a lambda to call a setter   The rank part won t work for an array of lists  but the sort will    First  here s  a pure list version my sortLambdaLst    lambda x y cmp x 0   y 0    lambda x y cmp x 1   y 1    def multi attribute sort x y       r   0     for l in my sortLambdaLst          r   l x y          if r  0  return r  keep looping till you see a difference     return r  Lst     4  2 0    4  0 01    4  0 9    4  0 999   4  0 2    1  2 0    1  0 01    1  0 9    1  0 999    1  0 2    Lst sort lambda x y multi attribute sort x y    The Lambda of the Lambda for rec in Lst  print str rec    Here s a way to rank a list of objects  class probe      def   init   self  group  score           self group   group         self score   score         self rank  -1     def set rank self  r           self rank   r     def   str   self           return   t  join  str self group   str self score   str self rank       def RankLst inLst  group lambda  lambda x x group  sortLambdaLst    lambda x y cmp x group  y group   lambda x y cmp x score  y score    SetRank Lambda   lambda x  rank x set rank rank         Inner function is the only way  I could think of  to pass the sortLambdaLst into a sort function     def multi attribute sort x y           r   0         for l in sortLambdaLst              r   l x y              if r  0  return r  keep looping till you see a difference         return r      inLst sort lambda x y multi attribute sort x y        Now Rank your probes     rank   0     last group   group lambda inLst 0       for i in range len inLst            rec   inLst i          group   group lambda rec          if last group    group               rank  1         else              rank 1             last group   group         SetRank Lambda inLst i   rank   This is pure evil   The lambda purists are gnashing their teeth  Lst    probe 4  2 0   probe 4  0 01   probe 4  0 9   probe 4  0 999   probe 4  0 2   probe 1  2 0   probe 1  0 01   probe 1  0 9   probe 1  0 999   probe 1  0 2     RankLst Lst  group lambda  lambda x x group  sortLambdaLst    lambda x y cmp x group  y group   lambda x y cmp x score  y score    SetRank Lambda   lambda x  rank x set rank rank   print   t  join   group    score    rank     for r in Lst  print r

User · Answer

A key can be a function that returns a tuple   s   sorted s  key   lambda x   x 1   x 2      Or you can achieve the same using itemgetter  which is faster and avoids a Python function call    import operator s   sorted s  key   operator itemgetter 1  2     And notice that here you can use sort instead of using sorted and then reassigning   s sort key   operator itemgetter 1  2

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I m not sure if this is the most pythonic method     I had a list of tuples that needed sorting 1st by descending integer values and 2nd alphabetically  This required reversing the integer sort but not the alphabetical sort  Here was my solution   on the fly in an exam btw  I was not even aware you could  nest  sorted functions   a      Al   2    Bill   1    Carol   2     Abel   3     Zeke   2     Chris   1     b   sorted sorted a  key   lambda x   x 0    key   lambda x   x 1   reverse   True    print b       Abel   3     Al   2     Carol   2     Zeke   2     Bill   1     Chris   1

[python] Sort a list by multiple attributes?

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