Sorting Lists in Python

Sorting a list in Python is a way to arrange the elements of the list in either ascending or descending order based on a defined criterion, such as numerical or lexicographical order.

This can be achieved using the built-in sorted() function or by calling the sort() method on the list itself, both of which modify the original list or return a new sorted list depending on the method used.

Sorting Lists Using sort() Method

The Python sort() method is used to sort the elements of a list in place. This means that it modifies the original list and does not return a new list.

Syntax

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

Where:

• list_name is the name of the list to be sorted.
• key (optional) is a function that defines the sorting criterion. If provided, it is applied to each element of the list for sorting. Default is None.
• reverse (optional) is a boolean value. If True, the list will be sorted in descending order. If False (default), the list will be sorted in ascending order.

Example of Sorting List in Lexicographical Order

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

Output:

List before sort: ['physics', 'Biology', 'chemistry', 'maths']
List after sort: ['Biology', 'chemistry', 'maths', 'physics']

Example of Sorting List in Numerical Order

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

Output:

List before sort: [10, 16, 9, 24, 5]
List after sort: [5, 9, 10, 16, 24]

Sorting Lists Using sorted() Method

The sorted() function in Python is a built-in function used to sort the elements of an iterable (such as a list, tuple, or string) and returns a new sorted list, leaving the original iterable unchanged.

Syntax

sorted(iterable, key=None, reverse=False)

Where:

• iterable is the iterable (e.g., list, tuple, string) whose elements are to be sorted.
• key (optional) is a function that defines the sorting criterion. If provided, it is applied to each element of the iterable for sorting. Default is None.
• reverse (optional) is a boolean value. If True, the iterable will be sorted in descending order. If False (default), the iterable will be sorted in ascending order.

Example

numbers = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]
# Sorting in descending order
sorted_numbers_desc = sorted(numbers, reverse=True)
print(sorted_numbers_desc)

Output:

[9, 6, 5, 5, 5, 4, 3, 3, 2, 1, 1]

Sorting List Items with Callback Function

In Python, a callback function refers to a function that is passed as an argument to another function and is invoked or called within that function.

We can sort list items with a callback function by using the sorted() function or sort() function in Python. Both of these functions allow us to specify a custom sorting criterion using the key parameter, which accepts a callback function.

Example Using str.lower() as key Parameter

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

Output:

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

Example Using User-Defined Function as key Parameter

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)

Output:

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