Python List sort() - Sorts Ascending or Descending List

The list.sort() method sorts the elements of a list in ascending or descending order using the default < comparisons operator between items.

Use the key parameter to pass the function name to be used for comparison instead of the default < operator. Set the reverse parameter to True, to get the list in descending order.

Syntax:

list.sort(key=None, reverse=False)

Parameters:

1. key: (Optional) A function that extracts a comparison key from each list element while sorting.
2. reverse: (Optional) If true, the sorted list will be reversed. By default, it is False.

Return Value:

No return value. It sorts the list itself.

The following example demonstrates the sort() function on numeric lists.

nums = [1, 5, 3, 4, 2, 10, 6, 8, 7, 9]
nums.sort()
print('List in Ascending Order: ', nums)

nums.sort(reverse=True)
print('List in Descending Order: ', nums)

List in Ascending Order: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
List in Descending Order: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

The sort() method can also be used on character lists.

al = ['a','d','e','c','b']
al.sort(reverse=True)
print('List in Descending Order: ', al)

al.sort()
print('List in Ascending Order: ', al)

List in Descending Order: ['e', 'd', 'c', 'b', 'a']
List in Ascending Order: ['a', 'b', 'c', 'd', 'e']

The following example sorts the string list in alphabetical order.

cities = ['Mumbai', 'London', 'Paris', 'New York']

cities.sort()
print('List in Ascending Order: ', cities)

cities.sort(reverse=True)
print('List in Descending Order: ', cities)

List in Ascending Order: ['London', 'Mumbai', 'New York', 'Paris']
List in Descending Order: ['Paris', 'New York', 'Mumbai', 'London']

Using key Parameter

Use the key parameter to set the built-in or custom function to compare each element of a list and sort it. For example, the following uses the built-in len() function that returns the length of each element and sorts based on the length of each element.

cities = ['Mumbai', 'London', 'Paris', 'New York']

cities.sort(key=len)
print('List in Ascending Order of the length: ', cities)

cities.sort(key=len, reverse=True)
print('List in Descending Order of the length: ', cities)

List in Ascending Order of the length: ['Paris', 'Mumbai', 'London', 'New York']
List in Descending Order of the length: ['New York', 'Mumbai', 'London', 'Paris']

Sort List of Class Objects

The following example shows how to sort a list whose elements are the objects of the custom class.

class student:
	name=''
	age=0
	def __init__(self, name, age):
		self.name = name
		self.age = age

s1 = student('Bill', 25)
s2 = student('Steve', 29)
s3 = student('Ravi', 26)

student_list = [s1, s2, s3]
# student_list.sort() # raise an error

student_list.sort(key=lambda s: s.name) # sorts using lambda function

print('Students in Ascending Order:', end=' ')

for std in student_list:
	print(std.name, end=', ')

student_list.sort(key=lambda s: s.name, reverse=True) # sorts using lambda function

print('Students in Descending Order:', end=' ')

for std in student_list:
	print(std.name, end=', ')

Students in Ascending Order: Bill, Ravi, Steve, 
Students in Descending Order: Steve, Ravi, Bill, 

In the above example, the lambda function lambda s: s.name set to the key argument. So, it will return the name of each student object for comparison. Direct use of student_list.sort() will raise an error because the < operator cannot compare objects. You can define a function instead of using the lambda function as a key argument.