Python reversed Function

Last modified April 11, 2025

This comprehensive guide explores Python's reversed function, which returns a reverse iterator for sequences. We'll cover built-in sequences, custom objects, and practical examples of reverse iteration.

Basic Definitions

The reversed function returns a reverse iterator object that accesses elements in reverse order. It works with any object that implements __reversed__ or supports sequence protocol (__len__ and __getitem__).

Key characteristics: returns an iterator (not a list), preserves original sequence, works with strings, lists, tuples, and custom sequences. It's memory efficient as it doesn't create a new sequence.

Basic Sequence Usage

Here's simple usage with different sequence types showing how reversed handles lists, tuples, and strings.

basic_reversed.py

# With lists
numbers = [1, 2, 3, 4, 5]
print(list(reversed(numbers)))  # [5, 4, 3, 2, 1]

# With tuples
colors = ('red', 'green', 'blue')
print(tuple(reversed(colors))) # ('blue', 'green', 'red')

# With strings
text = "hello"
print(''.join(reversed(text))) # "olleh"

This example shows reversed with different sequence types. Note that we convert the iterator to a concrete type (list, tuple, string) for display. The original sequences remain unchanged.

For strings, we use join to combine the reversed characters back into a string. The reversed function works with any sequence that supports indexing.

Custom Objects with reversed

You can make custom objects work with reversed by implementing the __reversed__ special method. This example creates a CountDown class.

custom_reversed.py

class CountDown:
    def __init__(self, start):
        self.start = start
    
    def __reversed__(self):
        n = 1
        while n <= self.start:
            yield n
            n += 1
    
    def __iter__(self):
        n = self.start
        while n > 0:
            yield n
            n -= 1

print("Normal iteration:")
for x in CountDown(5):
    print(x)  # 5, 4, 3, 2, 1

print("Reversed iteration:")
for x in reversed(CountDown(5)):
    print(x)  # 1, 2, 3, 4, 5

The CountDown class implements both __iter__ and __reversed__. Normal iteration counts down, while reversed iteration counts up.

This demonstrates how to customize reverse iteration behavior for your objects. The __reversed__ method should return an iterator.

Range Objects

The reversed function works efficiently with range objects, creating a reverse iterator without generating all numbers in memory.

range_reversed.py

# Forward range
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

# Reversed range
for i in reversed(range(5)):
    print(i)  # 4, 3, 2, 1, 0

# Large range (memory efficient)
large_range = reversed(range(1, 1000001))
print(next(large_range))  # 1000000
print(next(large_range))  # 999999

This example shows how reversed works with range objects. The reverse iteration is memory efficient, especially with large ranges.

The range object implements __reversed__ specially to provide this efficient reverse iteration without storing all numbers.

Error Handling

The reversed function raises TypeError when used with non-sequence types. This example shows proper error handling.

errors.py

try:
    print(list(reversed(42)))
except TypeError as e:
    print(f"Error: {e}")  # 'int' object is not reversible

class NoReverse:
    pass

try:
    print(list(reversed(NoReverse())))
except TypeError as e:
    print(f"Error: {e}")  # 'NoReverse' object is not reversible

These examples demonstrate reversed's behavior with unsupported types. Non-sequence objects and objects without __reversed__ or sequence protocol raise TypeError.

To make a class work with reversed, implement either __reversed__ or the sequence protocol as shown earlier.

Performance Considerations

This example compares reversed performance with alternative methods for reverse iteration.

performance.py

import timeit

def test_reversed():
    return list(reversed([1, 2, 3, 4, 5]))

def test_slice():
    return [1, 2, 3, 4, 5][::-1]

def test_manual():
    lst = [1, 2, 3, 4, 5]
    return [lst[i] for i in range(len(lst)-1, -1, -1)]

print("reversed():", timeit.timeit(test_reversed, number=1000000))
print("slice:", timeit.timeit(test_slice, number=1000000))
print("manual:", timeit.timeit(test_manual, number=1000000))

This benchmarks different reverse iteration methods. reversed is generally fastest for iteration. Slicing creates a new list but is fast for small sequences.

The manual approach is slower and less readable, demonstrating why reversed is preferred for iteration scenarios.

Best Practices

Use for iteration: Prefer reversed over slicing when you only need to iterate
Implement __reversed__: For custom sequence types that support reverse iteration
Consider slicing: When you need a reversed copy of the sequence
Handle errors: Catch TypeError when input type is uncertain
Document behavior: Clearly document __reversed__ implementation

Source References

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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