Why does return list sort return None not the list

Question

I ve been able to verify that the findUniqueWords does result in a sorted list  However  it does not return the list  Why   def findUniqueWords theList       newList          words             Read a line at a time     for item in theList             Remove any punctuation from the line         cleaned   cleanUp item             Split the line into separate words         words   cleaned split              Evaluate each word         for word in words                 Count each unique word             if word not in newList                  newList append word       answer   newList sort       return answer

User · Answer

The problem is here   answer   newList sort     sort does not return the sorted list  rather  it sorts the list in place   Use   answer   sorted newList

User · Answer

To understand why it does not return the list   sort   doesn t return any value while the sort   method just sorts the elements of a given list in a specific order - ascending or descending without returning any value   So problem is with answer   newList sort   where answer is none   Instead you can just do return  newList sort     The syntax of the sort   method is   list sort key      reverse        Alternatively  you can also use Python s in-built function sorted   for the same purpose   sorted list  key      reverse        Note  The simplest difference between sort   and sorted   is  sort   doesn t return any value while  sorted   returns an iterable list   So in your case answer   sorted newList

User · Answer

you can use sorted   method if you want it to return the sorted list    It s more convenient    l1      n   int input     for i in range n     user   int input      l1 append user  sorted l1 reverse True       list sort   method modifies the list in-place and returns None    if you still want to use sort you can do this    l1      n   int input     for i in range n     user   int input      l1 append user  l1 sort reverse True  print l1

User · Answer

Python has two kinds of sorts  a sort method  or  member function   and a sort function  The sort method operates on the contents of the object named -- think of it as an action that the object is taking to re-order itself  The sort function is an operation over the data represented by an object and returns a new object with the same contents in a sorted order   Given a list of integers named l the list itself will be reordered if we call l sort      gt  gt  gt  l    1  5  2341  467  213  123   gt  gt  gt  l sort    gt  gt  gt  l  1  5  123  213  467  2341    This method has no return value  But what if we try to assign the result of l sort      gt  gt  gt  l    1  5  2341  467  213  123   gt  gt  gt  r   l sort    gt  gt  gt  print r  None   r now equals actually nothing  This is one of those weird  somewhat annoying details that a programmer is likely to forget about after a period of absence from Python  which is why I am writing this  so I don t forget again    The function sorted    on the other hand  will not do anything to the contents of l  but will return a new  sorted list with the same contents as l    gt  gt  gt  l    1  5  2341  467  213  123   gt  gt  gt  r   sorted l   gt  gt  gt  l  1  5  2341  467  213  123   gt  gt  gt  r  1  5  123  213  467  2341    Be aware that the returned value is not a deep copy  so be cautious about side-effecty operations over elements contained within the list as usual    gt  gt  gt  spam    8  2  4  7   gt  gt  gt  eggs    3  1  4  5   gt  gt  gt  l    spam  eggs   gt  gt  gt  r   sorted l   gt  gt  gt  l   8  2  4  7    3  1  4  5    gt  gt  gt  r   3  1  4  5    8  2  4  7    gt  gt  gt  spam sort    gt  gt  gt  eggs sort    gt  gt  gt  l   2  4  7  8    1  3  4  5    gt  gt  gt  r   1  3  4  5    2  4  7  8

User · Answer

list sort sorts the list in place  i e  it doesn t return a new list  Just write  newList sort   return newList

User · Answer

Python habitually returns None from functions and methods that mutate the data  such as list sort  list append  and random shuffle  with the idea being that it hints to the fact that it was mutating   If you want to take an iterable and return a new  sorted list of its items  use the sorted builtin function

User · Answer

Here is an email from Guido van Rossum in Python s dev list explaining why he choose not to return self on operations that affects the object and don t return a new one      This comes from a coding style  popular in various other languages  I     believe especially Lisp revels in it  where a series of side effects     on a single object can be chained like this    x compress   chop y  sort z        which would be the same as    x compress     x chop y    x sort z        I find the chaining form a threat to readability  it requires that the       reader must be intimately familiar with each of the methods   The       second form makes it clear that each of these calls acts on the same       object  and so even if you don t know the class and its methods very       well  you can understand that the second and third call are applied to       x  and that all calls are made for their side-effects   and not to       something else       I d like to reserve chaining for operations that return new values        like string processing operations    y   x rstrip   n   split      lower

[python] Why does "return list.sort()" return None, not the list?

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