Sorting a Python list by two fields

Question

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

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python 3 https   docs python org 3 5 howto sorting html the-old-way-using-the-cmp-parameter from functools import cmp to key  def custom compare x  y         custom comparsion of x 0   x 1  with y 0   y 1      return 0  sorted entries  key lambda e   cmp to key custom compare  e 0    e 1

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list1   sorted csv1  key lambda x   x 1   x 2

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like this   import operator list1   sorted csv1  key operator itemgetter 1  2

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Sorting list of dicts using below will sort list in descending order on first column as salary and second column as age  d    salary  123  age  23    salary  123  age  25   d sorted d  key lambda i   i  salary    i  age    reverse True    Output     salary   123   age   25     salary   123   age   23

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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

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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

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In ascending order you can use   sorted data  sorted non sorted data  key lambda k   k 1  k 0      or in descending order you can use   sorted data  sorted non sorted data  key lambda k   k 1  k 0   reverse True

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employees sort key   lambda x x 1   employees sort key   lambda x x 0     We can also use  sort with lambda 2 times because python sort is in place and stable  This will first sort the list according to the second element  x 1   Then  it will sort the first element  x 0   highest priority    employees 0    Employee s Name employees 1    Employee s Salary   This is equivalent to doing the following  employees sort key   lambda x  x 0   x 1

[python] Sorting a Python list by two fields

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