Python reversed() — A Simple Guide with Video

Python’s built-in reversed(sequence) function returns a reverse iterator over the values of the given sequence such as a list, a tuple, or a string.

Python reversed()

Usage

Learn by example! Here are some examples of how to use the reversed() built-in function.

The most basic use is on a Python list:

>>> reversed([1, 2, 3])
<list_reverseiterator object at 0x0000017FB04EF8D0>

You can see that the return value of the reversed() function is not a list but an iterator object for efficiency reasons. There’s no reason to create a new reversed data structure just to go over a sequence in reverse order. Thus, if you want to create a new list or tuple of elements in reversed order, you need to convert the output to a list or tuple first using the list() or tuple() built-in functions.

>>> list(reversed([1, 2, 3]))
[3, 2, 1]
>>> tuple(reversed([1, 2, 3]))
(3, 2, 1)

You can also run the reversed() function on strings:

>>> print(*reversed('retxnif'))
f i n x t e r

This example makes use of the asterisk operator to unpack all elements in the reversed iterator into the print() function.

Syntax reversed()

You can use the reversed() method as follows:

Syntax:
reversed(sequence) -> new empty reversed iterator over sequence

Interactive Shell Exercise: Understanding reversed()

Consider the following interactive code:

Exercise: Guess the output before running the code.


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Performance Comparison of Ways to Reverse a List

There are many ways to reverse a list—all with linear runtime complexity in the number of elements. These are the primary methods to reverse a list:

  • list.reverse() — Best if you want to reverse the elements of list in place.
  • list[::-1] — Best if you want to write concise code to return a new list with reversed elements.
  • reversed(list) — Best if you want to iterate over all elements of a list in reversed order without changing the original list.

But, you may ask, what’s the most performant one?

Let’s run the reversed() method against different alternatives to reverse a list:

def reverse1():
    names = ['Alice', 'Bob', 'Carl', 'Dora']
    names.reverse()

def reverse2():
    names = ['Alice', 'Bob', 'Carl', 'Dora']
    names = names[::-1]

def reverse3():
    names = ['Alice', 'Bob', 'Carl', 'Dora']
    names = reversed(names)

def reverse4():
    names = ['Alice', 'Bob', 'Carl', 'Dora']
    l = []
    for i in range(len(names)-1, -1, -1):
        l.append(names[i])

def reverse5():
    names = ['Alice', 'Bob', 'Carl', 'Dora']
    l = []
    for i in range(1, len(names) + 1):
        l.append(names[-i])

import timeit
print('M1: ', timeit.timeit(reverse1, number=10000), '--> list.reverse()')
print('M2: ', timeit.timeit(reverse2, number=10000), '--> slicing')
print('M3: ', timeit.timeit(reverse3, number=10000), '--> reversed()')
print('M4: ', timeit.timeit(reverse4, number=10000), '--> loop')
print('M5: ', timeit.timeit(reverse5, number=10000), '--> loop + negative index')

The result is the following on my computer (Intel Core i7, 8th Gen, Python version 3.7):

M1:  0.0012140999999999957 --> list.reverse()
M2:  0.0016616999999999882 --> slicing
M3:  0.0019155999999999618 --> reversed()
M4:  0.005595399999999973 --> loop
M5:  0.006663499999999989 --> loop + negative index

It’s interesting to see that the least readable and least concise methods 4 and 5 are also slowest! Note that we didn’t convert the iterator returned by the reversed() method to a list—otherwise, it would have added a few milliseconds to the result.

Related Article: How to Reverse a List in Python?

Summary

Python’s built-in reversed(sequence) function returns a reverse iterator over the values of the given sequence such as a list, a tuple, or a string.

>>> list(reversed([1, 2, 3]))
[3, 2, 1]
>>> tuple(reversed([1, 2, 3]))
(3, 2, 1)

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