Python List [-1], [:n], [-n:], [::-1] Explained

You may see some code in Python like list[-1], list[:n], list[-n:], or list[::-1]. What do they mean? In this article, we will learn more about this technique which is called list slicing.

In simple words, the meaning of those expressions is:

• list[-1]: get the last element of the list
• list[:n]: get the first n elements of the list
• list[-n:]: get the last n elements of the list
• list[::-1]: reverse the list

Keep reading to understand more how list slicing works.

What is List Slicing?

In Python, list slicing is a technique to get a subset of a list using indices. The syntax is list[start:end:step] where:

• start: the index of the first element in the subset (inclusive), default to 0
• end: the index of the last element in the subset (exclusive), default to len(list)
• step: the step size to get elements in the subset, default to 1

For example, numbers[1:10:3] means to get all numbers from index 1 to 10 with a step size of 3.

Let's make that example more clear.

numbers = [2, 1, 0, 3, 5, 6, 8, 9, 7, 4]

print(numbers[1:10:3])
# Output: [1, 5, 9]

We have a list numbers with 10 elements. So numbers[1:10:3] are elements at 1, 4, and 7 indices, which are 1, 5, and 9, respectively.

What Does list[-1] Mean?

list[-1] means to get the last element of the list. It is equivalent to list[len(list) - 1].

numbers = [2, 1, 0, 3, 5, 6, 8, 9, 7, 4]

print(numbers[-1])
# Output: 4

What Does list[:n] Mean?

list[:n] means to get the first n elements of the list. It is equivalent to list[0:n].

If n is greater than the length of the list, it will return all elements of the list.

numbers = [2, 1, 0, 3, 5, 6, 8, 9, 7, 4]

print(numbers[:3])
# Output: [2, 1, 0]
print(numbers[:0])
# Output: []
print(numbers[:100])
# Output: [2, 1, 0, 3, 5, 6, 8, 9, 7, 4]

What Does list[-n:] Mean?

list[-n:] means to get the last n elements of the list. It is equivalent to list[len(list) - n:].

numbers = [2, 1, 0, 3, 5, 6, 8, 9, 7, 4]

print(numbers[-3:])
# Output: [9, 7, 4]
print(numbers[-5:])
# Output: [6, 8, 9, 7, 4]

What Does list[::-1] Mean?

list[::-1] means to reverse the list. It is equivalent to list[len(list) - 1:0:-1].

Let's break it down:

• len(list) - 1: the index of the last element of the list
• 0: the index of the first element of the list
• -1: the step size to get elements in the subset

So, in word, it means to go backward from the last element to the first element with a step size of 1.

numbers = [2, 1, 0, 3, 5, 6, 8, 9, 7, 4]

print(numbers[::-1])
# Output: [4, 7, 9, 8, 6, 5, 3, 0, 1, 2]