How do operator itemgetter and sort work

Question

I have the following code    initialize a         create the table  name  age  job  a append   quot Nick quot   30   quot Doctor quot    a append   quot John quot    8   quot Student quot    a append   quot Paul quot   22   quot Car Dealer quot    a append   quot Mark quot   66   quot Retired quot           sort the table by age import operator a sort key operator itemgetter 1          print the table print a   It creates a 4x3 table and then it sorts it by age  My question is  what exactly key operator itemgetter 1  does  Does the operator itemgetter function return the item s value  Why can t I just type something like key a x  1  there  Or can I  How could with operator print a certain value of the form like 3x2 which is 22   How does exactly Python sort the table  Can I reverse-sort it   How can I sort it based on two columns like first age  and then if age is the same b name   How could I do it without operator

User · Answer

a      a append   Nick   30   Doctor    a append   John    8   Student    a append   Paul    8  Car Dealer    a append   Mark   66   Retired    print a     Nick   30   Doctor      John   8   Student      Paul   8   Car Dealer      Mark   66   Retired     def  cmp a b             if a 1  lt b 1           return -1     elif a 1  gt b 1           return 1     else          return 0  sorted a cmp  cmp      John   8   Student      Paul   8   Car Dealer      Nick   30   Doctor      Mark   66   Retired     def  key list ele        return list ele 1   sorted a key  key      John   8   Student      Paul   8   Car Dealer      Nick   30   Doctor      Mark   66   Retired     gt  gt  gt

User · Answer

Answer for Python beginners  In simpler words     The key  parameter of sort requires a key function  to be applied to be objects to be sorted  rather than a single key value and  that is just what operator itemgetter 1  will give you  A function that grabs the first item from a list-like object      More precisely those are callables  not functions  but that is a difference that can often be ignored

User · Answer

You are asking a lot of questions that you could answer yourself by reading the documentation  so I ll give you a general advice  read it and experiment in the python shell  You ll see that itemgetter returns a callable    gt  gt  gt  func   operator itemgetter 1   gt  gt  gt  func a    Paul   22   Car Dealer    gt  gt  gt  func a 0   8   To do it in a different way  you can use lambda   a sort key lambda x  x 1     And reverse it   a sort key operator itemgetter 1   reverse True    Sort by more than one column   a sort key operator itemgetter 1 2     See the sorting How To

User · Answer

sorting first by age then profession you can change it in function  fun   a       def fun v       return  v 1  v 2      create the table  name  age  job  a append   Nick   30   Doctor    a append   John    8   Student    a append   Paul    8  Car Dealer    a append   Mark   66   Retired     a sort key fun    print a

User · Answer

Looks like you re a little bit confused about all that stuff  operator is a built-in module providing a set of convenient operators  In two words operator itemgetter n  constructs a callable that assumes an iterable object  e g  list  tuple  set  as input  and fetches the n-th element out of it  So  you can t use key a x  1  there  because python has no idea what x is  Instead  you could use a lambda function  elem is just a variable name  no magic there   a sort key lambda elem  elem 1    Or just an ordinary function  def get second elem iterable       return iterable 1   a sort key get second elem   So  here s an important note  in python functions are first-class citizens  so you can pass them to other functions as a parameter  Other questions   Yes  you can reverse sort  just add reverse True   a sort key      reverse True  To sort by more than one column you can use itemgetter with multiple indices  operator itemgetter 1 2   or with lambda  lambda elem   elem 1   elem 2    This way  iterables are constructed on the fly for each item in list  which are than compared against each other in lexicographic    order  first elements compared  if equal - second elements compared  etc  You can fetch value at  3 2  using a 2 1   indices are zero-based   Using operator    It s possible  but not as clean as just indexing   Refer to the documentation for details   operator itemgetter explained Sorting list by custom key in Python

[python] How do operator.itemgetter() and sort() work?

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