Custom Python list sorting

Question

I was refactoring some old code of mine and came across of this   alist sort cmp items   def cmp items a  b       if a foo  gt  b foo          return 1     elif a foo    b foo          return 0     else          return -1   The code works  and I wrote it some 3 years ago   but I cannot find this thing documented anywhere in the Python docs and everybody uses sorted   to implement custom sorting  Can someone explain why this works

User · Answer

As a side note  here is a better alternative to implement the same sorting   alist sort key lambda x  x foo    Or alternatively   import operator alist sort key operator attrgetter  foo      Check out the Sorting How To  it is very useful

User · Answer

It s documented here       The sort   method takes optional arguments for controlling the   comparisons       cmp specifies a custom comparison function of two arguments  list   items  which should  return a negative  zero or positive number   depending on whether the first argument is considered smaller than    equal to  or larger than the second argument  cmp lambda x y    cmp x lower    y lower     The default value is None

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I know many have already posted some good answers  However I want to suggest one nice and easy method without importing any library  l     2  3    3  4    2  4   l sort key   lambda x   -x 0   -x 1     print l  l sort key   lambda x   x 0   -x 1     print l   Output will be   3  4    2  4    2  3     2  4    2  3    3  4    The output will be sorted based on the order of the parameters we provided in the tuple format

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Even better  student tuples           john    A   15         jane    B   12         dave    B   10      sorted student tuples  key lambda student  student 2       sort by age    dave    B   10     jane    B   12     john    A   15    Taken from  https   docs python org 3 howto sorting html

User · Answer

This does not work in Python 3   You can use functools cmp to key to have old-style comparison functions work though   from functools import cmp to key  def cmp items a  b       if a foo  gt  b foo          return 1     elif a foo    b foo          return 0     else          return -1  cmp items py3   cmp to key cmp items   alist sort cmp items py3

User · Answer

Just like this example  You want sort this list     c   2     b   2     a   3    output     a   3     b   2     c   2    you should sort the tuples by the second item  then the first  def letter cmp a  b       if a 1   gt  b 1           return -1     elif a 1     b 1           if a 0   gt  b 0               return 1         else              return -1     else          return 1  Then convert it to a key function  from functools import cmp to key letter cmp key   cmp to key letter cmp    Now you can use your custom sort order     c   2     b   2     a   3   sort key letter cmp key

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