Python - Sort Lists

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The sort() method of list class rearranges the items in ascending or descending order with the use of lexicographical ordering mechanism. The sorting is in-place, in the sense the rearrangement takes place in the same list object, and that it doesn't return a new object.

Syntax

list1.sort(key, reverse)

Parameters

• Key − The function applied to each item in the list. The return value is used to perform sort. Optional

• reverse − Boolean value. If set to True, the sort takes place in descending order. Optional

Return value

This method returns None.

Example 1

Now let's take a look at some examples to understand how we can sort lists in Python −

list1 = ['physics', 'Biology', 'chemistry', 'maths']
print ("list before sort", list1)
list1.sort()
print ("list after sort : ", list1)

print ("Descending sort")

list2 = [10,16, 9, 24, 5]
print ("list before sort", list2)
list2.sort()
print ("list after sort : ", list2)

It will produce the following output −

list before sort ['physics', 'Biology', 'chemistry', 'maths']
list after sort: ['Biology', 'chemistry', 'maths', 'physics']
Descending sort
list before sort [10, 16, 9, 24, 5]
list after sort : [5, 9, 10, 16, 24]

Example 2

In this example, the str.lower() method is used as key parameter in sort() method.

list1 = ['Physics', 'biology', 'Biomechanics', 'psychology']
print ("list before sort", list1)
list1.sort(key=str.lower)
print ("list after sort : ", list1)

It will produce the following output −

list before sort ['Physics', 'biology', 'Biomechanics', 'psychology']
list after sort : ['biology', 'Biomechanics', 'Physics', 'psychology']

Example 3

Let us use a user-defined function as the key parameter in sort() method. The myfunction() uses % operator to return the remainder, based on which the sort is done.

def myfunction(x):
   return x%10
list1 = [17, 23, 46, 51, 90]
print ("list before sort", list1)
list1.sort(key=myfunction)
print ("list after sort : ", list1)

It will produce the following output −

list before sort [17, 23, 46, 51, 90]
list after sort: [90, 51, 23, 46, 17]