Python reversed vs reverse: Key Differences

Python provides two distinct ways to reverse sequences: the built-in reversed() function and the list method .reverse(). While both achieve a reversed order, they differ fundamentally in how they operate on data, what they return, their memory usage, and the data types they support. This article explains how reversed() creates a memory-efficient iterator without modifying the original data, while .reverse() mutates a list in place and returns None, allowing you to choose the correct tool for your specific use case.

1. In-Place Mutation vs. Non-Destructive Operation

The most critical distinction between these two options is whether the original object is modified.

• list.reverse() is an in-place operation. It rearranges the elements directly inside the original list object in memory, permanently changing its order.
• reversed() is non-destructive. It leaves the original sequence untouched and generates a new sequence iterator.

# Using .reverse()
numbers = [1, 2, 3]
numbers.reverse()
print(numbers)  # Output: [3, 2, 1] (original list is modified)

# Using reversed()
numbers = [1, 2, 3]
rev_iterator = reversed(numbers)
print(numbers)  # Output: [1, 2, 3] (original list remains unchanged)

2. Return Values

Because their operations differ, their return values are completely different:

• list.reverse() returns None: Attempting to assign the result of .reverse() to a variable will store None. This is a common pitfall for beginners.
• reversed() returns an iterator: It returns a specialized reverse iterator object (e.g., list_reverseiterator or reversed object) that yields elements one by one upon request.

data = [10, 20, 30]

# .reverse() returns None
result = data.reverse()
print(result)  # Output: None

# reversed() returns an iterator
rev_obj = reversed([10, 20, 30])
print(rev_obj)  # Output: <list_reverseiterator object at 0x...>
print(list(rev_obj))  # Output: [30, 20, 10]

3. Supported Data Types

• list.reverse() is exclusively a list method: It cannot be used on tuples, strings, ranges, or dictionaries.
• reversed() works on any sequence: It accepts any iterable that implements the __reversed__() method or supports the sequence protocol (implements __len__() and integer-based __getitem__()).

# reversed() on a string
name = "Python"
print("".join(reversed(name)))  # Output: "nohtyP"

# reversed() on a tuple
coords = (1, 2, 3)
print(tuple(reversed(coords)))  # Output: (3, 2, 1)

# .reverse() fails on strings or tuples
# "Python".reverse() -> AttributeError: 'str' object has no attribute 'reverse'

4. Memory and Performance

• list.reverse() operates with \(O(1)\) auxiliary space complexity because it swaps elements within the existing array without allocating additional memory for elements.
• reversed() also uses \(O(1)\) auxiliary memory because it evaluates lazily. It does not create a full reversed copy in memory; instead, it tracks an index pointer backwards through the existing sequence.

However, if you convert the iterator produced by reversed() into a concrete list using list(reversed(seq)), it requires \(O(n)\) space to store the new list.

Comparison Summary

When to Use Which

• Use .reverse() when:

  • You are working strictly with a list.
  • You do not need to preserve the original order of the list.
  • You want to avoid the minor overhead of instantiating an iterator.

• Use reversed() when:

  • You need to keep the original collection intact.
  • You are working with immutable sequences such as strings or tuples.
  • You only need to iterate over items in reverse order inside a for loop without allocating a new list in memory:

# Efficient loop without mutating data or creating new lists
items = ["a", "b", "c"]
for item in reversed(items):
    print(item)