PythonArraysIntermediate

reversed()

Returns an iterator that yields items in reverse order (no copy).

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it = reversed(sequence)
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What it does

Overview

Returns an iterator that yields items in reverse order (no copy).

The reversed() built-in function returns a reverse iterator for a given sequence. Unlike the list.reverse() method, which modifies a list in-place and returns None, reversed() creates a new iterator object that yields elements in reverse order without duplicating the original data. This is highly memory-efficient (O(1) auxiliary space complexity) because it doesn't create a shallow or deep copy of the sequence. To use reversed(), the target object must either implement the __reversed__() magic method or support the sequence protocol (containing both __len__() and __getitem__() methods). Note that because it returns an iterator, the result is 'lazy' and can only be consumed once. To convert the result back into a list, string, or tuple, you must explicitly pass the iterator to a constructor like list() or join it with a string.

Quick reference

Syntax

it = reversed(sequence)

Inputs

Parameters

sequenceSequence[Any] · A sequence that supports reverse iteration.

See it in practice

Examples

1

Reversing a List into a New List

items = ['apple', 'banana', 'cherry']
reversed_items = list(reversed(items))
print(reversed_items)
Output:
['cherry', 'banana', 'apple']

Uses the list() constructor to consume the reverse iterator and create a new list object.

2

Reversing a String

text = 'Python'
reversed_text = ''.join(reversed(text))
print(reversed_text)
Output:
nohtyP

Strings are sequences. Since reversed() returns an iterator, the join() method is used to concatenate the characters back into a string.

3

Memory-Efficient Iteration

for num in reversed(range(1, 4)):
    print(num)
Output:
3 2 1

Iterates directly over the reverse iterator, which is memory-efficient for large ranges or lists as no intermediate copy is made.

Debug faster

Common Errors

1

TypeError

Cause: Attempting to reverse an object that does not support the sequence protocol (like a set or a basic dictionary).

Fix: Convert the collection to a list first, or use a collection type that maintains order and supports indexing.

reversed({1, 2, 3})  # Raises TypeError: 'set' object is not reversible
2

LogicError

Cause: Attempting to access elements by index or check the length of the result of reversed() directly.

Fix: Cast the result to a list() if indexing or multiple passes are required.

rev = reversed([1, 2, 3])
print(rev[0])  # Raises TypeError: 'list_reverseiterator' object is not subscriptable

Runtime support

Compatibility

Python 3.8+

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Returns an iterator that yields items in reverse order (no copy).

sequence: A sequence that supports reverse iteration.

TypeError: Convert the collection to a list first, or use a collection type that maintains order and supports indexing. LogicError: Cast the result to a list() if indexing or multiple passes are required.