How to sort a list of objects based on an attribute of the objects

Question

I ve got a list of Python objects that I d like to sort by an attribute of the objects themselves   The list looks like    gt  gt  gt  ut   lt Tag  128 gt    lt Tag  2008 gt    lt Tag   lt  gt    lt Tag  actionscript gt    lt Tag  addresses gt     lt Tag  aes gt    lt Tag  ajax gt         Each object has a count    gt  gt  gt  ut 1  count 1L   I need to sort the list by number of counts descending   I ve seen several methods for this  but I m looking for best practice in Python

User · Answer

A way that can be fastest  especially if your list has a lot of records  is to use operator attrgetter  count    However  this might run on an pre-operator version of Python  so it would be nice to have a fallback mechanism  You might want to do the following  then   try  import operator except ImportError  keyfun  lambda x  x count   use a lambda if no operator module else  keyfun  operator attrgetter  count     use operator since it s faster than lambda  ut sort key keyfun  reverse True    sort in-place

User · Answer

To sort the list in place    ut sort key lambda x  x count  reverse True     To return a new list  use the sorted   built-in function    newlist   sorted ut  key lambda x  x count  reverse True    More on sorting by keys

User · Answer

Add rich comparison operators to the object class  then use sort   method of the list  See rich comparison in python     Update  Although this method would work  I think solution from Triptych is better suited to your case because way simpler

User · Answer

A way that can be fastest  especially if your list has a lot of records  is to use operator attrgetter  count    However  this might run on an pre-operator version of Python  so it would be nice to have a fallback mechanism  You might want to do the following  then   try  import operator except ImportError  keyfun  lambda x  x count   use a lambda if no operator module else  keyfun  operator attrgetter  count     use operator since it s faster than lambda  ut sort key keyfun  reverse True    sort in-place

User · Answer

A way that can be fastest  especially if your list has a lot of records  is to use operator attrgetter  count    However  this might run on an pre-operator version of Python  so it would be nice to have a fallback mechanism  You might want to do the following  then   try  import operator except ImportError  keyfun  lambda x  x count   use a lambda if no operator module else  keyfun  operator attrgetter  count     use operator since it s faster than lambda  ut sort key keyfun  reverse True    sort in-place

User · Answer

If the attribute you want to sort by is a property  then you can avoid importing operator attrgetter and use the property s fget method instead     For example  for a class Circle with a property radius we could sort a list of circles by radii as follows   result   sorted circles  key Circle radius fget    This is not the most well-known feature but often saves me a line with the import

User · Answer

from operator import attrgetter ut sort key   attrgetter  count    reverse   True

User · Answer

Add rich comparison operators to the object class  then use sort   method of the list  See rich comparison in python     Update  Although this method would work  I think solution from Triptych is better suited to your case because way simpler

User · Answer

A way that can be fastest  especially if your list has a lot of records  is to use operator attrgetter  count    However  this might run on an pre-operator version of Python  so it would be nice to have a fallback mechanism  You might want to do the following  then   try  import operator except ImportError  keyfun  lambda x  x count   use a lambda if no operator module else  keyfun  operator attrgetter  count     use operator since it s faster than lambda  ut sort key keyfun  reverse True    sort in-place

User · Answer

Add rich comparison operators to the object class  then use sort   method of the list  See rich comparison in python     Update  Although this method would work  I think solution from Triptych is better suited to your case because way simpler

User · Answer

Readers should notice that the key  method   ut sort key lambda x  x count  reverse True    is many times faster than adding rich comparison operators to the objects  I was surprised to read this  page 485 of  Python in a Nutshell    You can confirm this by running tests on this little program      usr bin env python import random  class C      def   init   self count           self count   count      def   cmp   self other           return cmp self count other count   longList    C random random    for i in xrange 1000000    about 6 1 secs longList2   longList     longList sort    about 52 - 6 1   46 secs longList2 sort key   lambda c  c count   about 9 - 6 1   3 secs   My  very minimal  tests show the first sort is more than 10 times slower  but the book says it is only about 5 times slower in general  The reason they say is due to the highly optimizes sort algorithm used in python  timsort    Still  its very odd that  sort lambda  is faster than plain old  sort    I hope they fix that

User · Answer

It looks much like a list of Django ORM model instances   Why not sort them on query like this   ut   Tag objects order by  -count

User · Answer

from operator import attrgetter ut sort key   attrgetter  count    reverse   True

User · Answer

To sort the list in place    ut sort key lambda x  x count  reverse True     To return a new list  use the sorted   built-in function    newlist   sorted ut  key lambda x  x count  reverse True    More on sorting by keys

User · Answer

It looks much like a list of Django ORM model instances   Why not sort them on query like this   ut   Tag objects order by  -count

User · Answer

If the attribute you want to sort by is a property  then you can avoid importing operator attrgetter and use the property s fget method instead     For example  for a class Circle with a property radius we could sort a list of circles by radii as follows   result   sorted circles  key Circle radius fget    This is not the most well-known feature but often saves me a line with the import

User · Answer

Readers should notice that the key  method   ut sort key lambda x  x count  reverse True    is many times faster than adding rich comparison operators to the objects  I was surprised to read this  page 485 of  Python in a Nutshell    You can confirm this by running tests on this little program      usr bin env python import random  class C      def   init   self count           self count   count      def   cmp   self other           return cmp self count other count   longList    C random random    for i in xrange 1000000    about 6 1 secs longList2   longList     longList sort    about 52 - 6 1   46 secs longList2 sort key   lambda c  c count   about 9 - 6 1   3 secs   My  very minimal  tests show the first sort is more than 10 times slower  but the book says it is only about 5 times slower in general  The reason they say is due to the highly optimizes sort algorithm used in python  timsort    Still  its very odd that  sort lambda  is faster than plain old  sort    I hope they fix that

User · Answer

Object-oriented approach  It s good practice to make object sorting logic  if applicable  a property of the class rather than incorporated in each instance the ordering is required   This ensures consistency and removes the need for boilerplate code    At a minimum  you should specify   eq   and   lt   operations for this to work  Then just use sorted list of objects    class Card object        def   init   self  rank  suit           self rank   rank         self suit   suit      def   eq   self  other           return self rank    other rank and self suit    other suit      def   lt   self  other           return self rank  lt  other rank  hand    Card 10   H    Card 2   h    Card 12   h    Card 13   h    Card 14   h    hand order    c rank for c in hand      10  2  12  13  14   hand sorted   sorted hand  hand sorted order    c rank for c in hand sorted      2  10  12  13  14

User · Answer

It looks much like a list of Django ORM model instances   Why not sort them on query like this   ut   Tag objects order by  -count

User · Answer

Object-oriented approach  It s good practice to make object sorting logic  if applicable  a property of the class rather than incorporated in each instance the ordering is required   This ensures consistency and removes the need for boilerplate code    At a minimum  you should specify   eq   and   lt   operations for this to work  Then just use sorted list of objects    class Card object        def   init   self  rank  suit           self rank   rank         self suit   suit      def   eq   self  other           return self rank    other rank and self suit    other suit      def   lt   self  other           return self rank  lt  other rank  hand    Card 10   H    Card 2   h    Card 12   h    Card 13   h    Card 14   h    hand order    c rank for c in hand      10  2  12  13  14   hand sorted   sorted hand  hand sorted order    c rank for c in hand sorted      2  10  12  13  14

User · Answer

It looks much like a list of Django ORM model instances   Why not sort them on query like this   ut   Tag objects order by  -count

User · Answer

from operator import attrgetter ut sort key   attrgetter  count    reverse   True

User · Answer

Add rich comparison operators to the object class  then use sort   method of the list  See rich comparison in python     Update  Although this method would work  I think solution from Triptych is better suited to your case because way simpler

User · Answer

To sort the list in place    ut sort key lambda x  x count  reverse True     To return a new list  use the sorted   built-in function    newlist   sorted ut  key lambda x  x count  reverse True    More on sorting by keys

User · Answer

To sort the list in place    ut sort key lambda x  x count  reverse True     To return a new list  use the sorted   built-in function    newlist   sorted ut  key lambda x  x count  reverse True    More on sorting by keys

User · Answer

from operator import attrgetter ut sort key   attrgetter  count    reverse   True

[python] How to sort a list of objects based on an attribute of the objects?

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