Python reverse

last modified July 6, 2020

In this tutorial we show how to reverse Python sequences.

When we reverse items, we change their order. Note that reversing should not be confused with sorting in descending order.

A Python list has a reverse() function. The [::-1] slice operation to reverse a Python sequence. The reversed() built-in function returns a reverse iterator. The object's __reversed__() magic method is called by the reversed() built-in to implement reverse iteration.

Python reverse list

In the first example, we reverse a Python list with the reverse() method and the [::-1] operator.

#!/usr/bin/env python3

nums = [2, 7, 8, 9, 1, 0]
nums.reverse()

print(nums)

rev_nums = nums[::-1]
print(rev_nums)

The reverse() method reverses the list in place. The nums[::-1] creates a new copy of the list where the elements are reversed.

$ ./reverse_list.py
[0, 1, 9, 8, 7, 2]

This is the output.

Python reversed function

The reversed() built-in function returns a reverse iterator.

#!/usr/bin/env python3

words = ['forest', 'wood', 'sky', 'rock']

for word in reversed(words):
    print(word)

word = 'forest'

for e in reversed(word):
    print(e, end=' ')

print()

for e in reversed(range(1, 10, 2)):
    print(e)

In the example, we use the reversed() function on a list, word, and a range.

$ ./reversed_fun.py
rock
sky
wood
forest
t s e r o f
9
7
5
3
1

This is the output.

Python custom reverse string function

In the following example, we create a custom string reverse function.

#!/usr/bin/env python3

def reverse_string(word):

    rev = ''
    n = len(word)

    while n > 0:
        n -= 1                 
        rev += word[n] 
    return rev

word = 'forest'

print(reverse_string('forest'))

Note that this is for demonstrational purposes; this implementation is slow.

def reverse_string(word):

    rev = ''
    n = len(word)

    while n > 0:
        n -= 1                 
        rev += word[n] 
    return rev

In the function, we use a while loop to build the new string in reverse order.

Python __reversed__ method

The __reversed__() magic method implementation should return a new iterator object that iterates over all the objects in the container in reverse order.

#!/usr/bin/env python3

class Vowels(object):

    def __init__(self):

        self.vowels = ['a', 'e', 'i', 'o', 'u', 'y']

    def __len__(self):
        return len(self.vowels)

    def __getitem__(self, e):
        return self.vowels[e]

    def __reversed__(self):
        for e in self.vowels[::-1]:
            yield elem        

vowels = Vowels()

print('normal order:')
for vowel in vowels:
    print(vowel, end=' ')

print()

print('reversed order:')
for vowel in reversed(vowels):
    print(vowel, end=' ')    

print()   

In the example, we implement the __reversed__() method in a Vowels object.

$ ./reversed_magic.py
normal order:
a e i o u y
reversed order:
y u o i e a

This is the output.

In this tutorial, we have done reversing operations in Python.

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