Reverse A List

Introduction

Python is a flexible language, and there's typically several ways to perform the same, menial task. Choosing an approach can depend on the time or space complexity, or simply on your personal preference.

Python's data structures are quite handy and intuitive, and their built-in functionalities are easy to work with. In this article, we'll be looking at how to reverse a list in Python. A Python List is a heterogenous (can contain differing types) array-like structure that stores references to objects in memory.

When manipulating a list, we can either create a new, changed list, or change the original list in-place. We'll see the differences in these as we proceed through the article.

Reverse a List Using the reverse() Method

Python has a powerful built-in library of methods when it comes to manipulating data in data structures. For the purposes of reversing a list, we can utilize the built-in reverse() method.

Note: The reverse() method reverses the list in-place. Reversing a list in-place means that the original list is changed, instead of creating a new, reversed list.

Due to this, we can't assign the resulting object to a new variable, and if you want to keep the original list in memory, you'll have to copy it before reversing:

my_list = [1, 2, 3, 4]
my_list.reverse()
print(my_list) # Output: [4, 3, 2, 1]

new_list = my_list.reverse()
print(new_list) # Output: None

There's no return value - the list is reversed in-place. However, we can copy() it before reversing:

list_1 = [1, 2, 3, 4]
list_2 = list_1.copy()

list_1.reverse()

print('Reversed list: ', list_1)
print('Saved original list: ', list_2)

This results in

Reversed list:  [4, 3, 2, 1]
Saved original list:  [1, 2, 3, 4]

Reverse a List Using Slice Notation

The slice notation allows us to slice and reproduce parts of various collections or collection-based objects in Python, such as Lists, Strings, Tuples and NumPy Arrays.

When you slice a list, a portion is returned from that list, and every stepth element is included:

my_list = [1, 2, 3, 4, 5, 6]

# list[start:end:step]
segment_1 = my_list[1:5:1]
segment_2 = my_list[1:5:2]

print(segment_1)
print(segment_2)

This results in:

[2, 3, 4, 5]
[2, 4]

By omitting the start and end arguments, you can include the entire collection. And by setting the step to a negative number, you iterate through the collection in reverse. Naturally, if you pair these together:

original_list = [1, 2, 3, 4, 5, 6]

reversed_list = original_list[::-1]
print('Original list: ', original_list)
print('Reversed list: ', reversed_list)

This results in:

Original list:  [1, 2, 3, 4, 5, 6]
Reversed list:  [6, 5, 4, 3, 2, 1]

The Slice Notation doesn't affect the original list at all, so the original_list stays the same even after the operation.

Reverse a List Using slice() Method

The slice() method accepts the very same parameters - start, end and step, and performs much the same operation as the Slice Notation. Though, instead of omitting the start and end arguments, you can pass in None.

Its return type is a Slice object, which can be then be used to slice a collection according to its contents. It's not called on the collection you're slicing - you're passing in the Slice object after creation, allowing you to create a single reusable and callable object for many different collections.

It's internally transpiled into Slice Notation, so the end result is the same:

original_list = [1, 2, 3, 4, 5, 6]

slice_obj = slice(None, None, -1)

print('slice_obj type:', type(slice_obj))
print('Reversed list:', original_list[slice_obj])

This results in:

slice_obj type: <class 'slice'>
Reversed list: [6, 5, 4, 3, 2, 1]

Reverse a List Using a For Loop

Depending on whether we want to keep the original list intact or not, we can pop() elements from the original list and add them to a new one, or we can just append them in reverse order.

pop() removes the last element from a collection and returns it. We can combine the append() method with this to directly append the removed element to a new list, effectively resulting in a reversed list:

original_list = [1, 2, 3, 4] 
reversed_list = []

for i in range(len(original_list)):
    reversed_list.append(original_list.pop())

print(reversed_list) # Output: [4, 3, 2, 1]

Alternatively, we can iterate through the list backwards, until the -1th index (non-inclusive) and add each element we see along that list. The range() method accepts 3 arguments - start, end and step, which can again be used in much the same way as before:

original_list = [1, 2, 3, 4]
reversed_list = []

for i in range(len(original_list)-1, -1, -1):
    reversed_list.append(original_list[i])

print(reversed_list) # Output: [4, 3, 2, 1]

Reversing a List Using the reversed() Method

Since iterating with a negative step and then accessing each element in the original list is a bit verbose, the reversed() method was added, which makes it much easier to manually implement the reversal logic, in case you want to add your own twist on it.

The reversed() method returns an iterator, iterating over the collection in a reversed order - and we can easily add these elements into a new list:

original_list = [1, 2, 3, 4]
new_list = []

for i in reversed(original_list):
	new_list.append(i)
    
print(new_list) # Output: [4, 3, 2, 1]
print(original_list) # Output: [1, 2, 3, 4] --> Original hasn't changed

Conclusion

Depending on whether you need a new reversed list, an in-place reversed list, as well as whether you want the logic to be taken care of for you, or if you'd like to have the flexibility of adding additional operations or twists during the reversal - there are several ways to reverse a list in Python.

In this tutorial, we've gone over these scenarios, highlighting the difference between each.