Python List Slice

Summary: in this tutorial, you’ll learn about Python list slice and how to use it to manipulate lists effectively.

Introduction to Python List slice notation

Lists support the slice notation that allows you to get a sublist from a list:

sub_list = list[begin: end: step]Code language: Python (python)

In this syntax, the begin, end, and step arguments must be valid indexes. And they’re all optional.

The begin index defaults to zero. The end index defaults to the length of the list. And the step index defaults to 1.

The slice will start from the begin up to the end in the step of step.

The begin, end, and step can be positive or negative. Positive values slice the list from the first element to the last element while negative values slice the list from the last element to the first element.

In addition to extracting a sublist, you can use the list slice to change the list such as updating, resizing, and deleting a part of the list.

Python List slice examples

Let’s take some examples of using the list slice.

1) Basic Python list slice example

Suppose that you have the following list of strings:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']Code language: Python (python)

The following example uses the list slice to get a sublist from the colors list:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
sub_colors = colors[1:4]

print(sub_colors)
Code language: Python (python)

Output:

['orange', 'yellow', 'green']Code language: Python (python)

The begin index is 1, so the slice starts from the 'orange' color. The end index is 4, therefore, the last element of the slice is 'green'.

As a result, the slice creates a new list with three colors: ['orange', 'yellow', 'green'].

This example doesn’t use the step, so the slice gets all values within the range without skipping any elements.

2) Using Python List slice to get the n-first elements from a list

To get the n-first elements from a list, you omit the first argument:

list[:n]Code language: Python (python)

The following example returns a list that includes the first three elements from the colors list:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
sub_colors = colors[:3]

print(sub_colors)
Code language: Python (python)

Output:

['red', 'orange', 'yellow']Code language: Python (python)

Notice that the colors[:3] is equivalent to the color[0:3].

3) Using Python List slice to get the n-last elements from a list

To get the n-last elements of a list, you use the negative indexes.

For example, the following returns a list that includes the last 3 elements of the colors list:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
sub_colors = colors[-3:]

print(sub_colors)
Code language: Python (python)

Output:

['blue', 'indigo', 'violet']Code language: Python (python)

4) Using Python List slice to get every nth element from a list

The following example uses the step to return a sublist that includes every 2nd element of the colors list:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
sub_colors = colors[::2]

print(sub_colors)
Code language: Python (python)

Output:

['red', 'yellow', 'blue', 'violet']Code language: Python (python)

5) Using Python List slice to reverse a list

When you use a negative step, the slice includes the list of elements starting from the last element to the first element. In other words, it reverses the list. See the following example:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
reversed_colors = colors[::-1]

print(reversed_colors)
Code language: Python (python)

Output:

['violet', 'indigo', 'blue', 'green', 'yellow', 'orange', 'red']Code language: Python (python)

6) Using Python List slice to substitute part of a list

Besides extracting a part of a list, the list slice allows you to change the list element.

The following example changes the first two elements in the colors list to the new values:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
colors[0:2] = ['black', 'white']

print(colors)
Code language: Python (python)

Output:

['black', 'white', 'yellow', 'green', 'blue', 'indigo', 'violet']Code language: Python (python)

7) Using Python List slice to partially replace and resize a list

The following example uses the list slice to replace the first and second elements with the new ones and also add a new element to the list:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
print(f"The list has {len(colors)} elements")

colors[0:2] = ['black', 'white', 'gray']
print(colors)
print(f"The list now has {len(colors)} elements")
Code language: Python (python)

Output:

The list has 7 elements
['black', 'white', 'gray', 'yellow', 'green', 'blue', 'indigo', 'violet']
The list now has 8 elementsCode language: Python (python)

8) Using Python list slice to delete elements

The following shows how to use the list slice to delete the 3rd, 4th, and 5th elements from the colors list:

colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
del colors[2:5]

print(colors)
Code language: Python (python)

Output:

['red', 'orange', 'indigo', 'violet']Code language: Python (python)

Summary

• Use a list slice to extract a sublist from a list and modify the list.