How to Fix – IndexError list assignment index out of range

The IndexError is a common error that occurs in Python when you try to access an index that is out of range for a list. This error can occur when you try to assign a value to an index that does not exist in the list. In this tutorial, we will discuss how to fix the IndexError list assignment index out of range in Python.

Understanding the Error

Before we dive into the solution, let’s first understand what causes the IndexError list assignment index out of range error. This error occurs when you try to assign a value to an index that does not exist in the list. For example, consider the following code:

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

Output:

---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

Cell In[1], line 2
      1 my_list = [1, 2, 3]
----> 2 my_list[3] = 4

IndexError: list assignment index out of range

In the above code, we are trying to assign the value 4 to the index 3 of the my_list list. However, the my_list list only has three elements, so the index 3 does not exist. This results in the IndexError list assignment index out of range error.

Fixing the Error

To fix the IndexError list assignment index out of range error, you need to make sure that the index you are trying to access or assign a value to exists in the list. You can use an if statement to check if the index exists in the list.

Note that Python sequences also allow negative indexing so be mindful of this when checking if an index exists in a list or not. Let’s look at an example.

# create a list
my_list = [1, 2, 3]
# index to assign the value
index = 3
# check if the index exits, if yes, then assign the value
if -len(my_list) <= index < len(my_list):
    # perform the assignment operation
    my_list[3] = 4
else:
    print("Assingnment index is out of range")

Output:

Assingnment index is out of range

Note that if you’re sure that the given index is not negative, you can just check if the index lies in the range 0 to the length of the list using index < len(my_list).

If your end goal is to add an element to a list, you can instead use the list append() or insert() functions.

The append() method adds an element to the end of the list, so you don’t have to worry about the index. For example:

my_list = [1, 2, 3]
my_list.append(4)
print(my_list)

Output:

[1, 2, 3, 4]

Here, we added the element 4 to the end of the list and we didn’t need to provide an index.

If you need to add an element to a specific index in the list, you can use the insert() method. The insert() method takes two arguments: the index where you want to insert the element and the element itself.

my_list = [1, 2, 3]
# insert the element 4 at index 3
my_list.insert(3, 4)
print(my_list)

Output:

[1, 2, 3, 4]

In this code, we are using the insert() method to insert the value 4 at the index 3 of the my_list list. Since we are inserting the element at a specific index, the IndexError list assignment index out of range error will be avoided.

Conclusion

The IndexError list assignment index out of range error occurs when you try to assign a value to an index that does not exist in the list. To fix this error, you need to make sure that the index you are trying to access or assign a value to exists in the list. You can do this by checking the length of the list. If you want to add values to a list, you can use the append() method to add elements to the end of the list, or the insert() method to insert elements at a specific index.

You might also be interested in –

• Understand and Fix IndexError in Python
• Python List Append, Extend and Insert