In this article you'll see a few of the reasons that cause the list index out of range Python error.
Besides knowing why this error occurs in the first place, you'll also learn some ways to avoid it.
Let's get started!
How to Create a List in Python
To create a list object in Python, you need to:
- Give the list a name,
- Use the assignment operator,
=, - and include 0 or more list items inside square brackets,
[]. Each list item needs to be separated by a comma.
For example, to create a list of names you would do the following:
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
The code above created a list called names that has four values: Kelly, Nelly, Jimmy, Lenny.
How to Check the Length of a List in Python
To check the length of a list in Python, use Python's build-in len() method.
len() will return an integer, which will be the number of items stored in the list.
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
#create a variable called name_length to store the length of the list
name_length = len(names)
#print value of variable to the console
print(name_length)
#output
#4
There are four items stored in the list, therefore the length of the list will be four.
How to Access Individual List Items in Python
Each item in a list has its own index number.
Indexing in Python, and most modern programming languages, starts at 0.
This means that the first item in a list has an index of 0, the second item has an index of 1, and so on.
You can use the index number to access the individual item.
To access an item in a list using its index number, first write the name of the list. Then, inside square brackets, include the intiger that corresponds with the item's index number.
Taking the example from earlier, this is how you would access each item inside the list using its index number:
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
names[0] # Kelly
names[1] # Nelly
names[2] # Jimmy
names[3] # Lenny
You can also use negative indexing to access items inside lists in Python.
To access the last item, you use the index value of -1. To acces the second to last item, you use the index value of -2.
Here is how you would access each item inside a list using negative indexing:
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
names[-4] # Kelly
names[-3]# Nelly
names[-2] # Jimmy
names[-1] # Lenny
Why does the Indexerror: list index out of range error occur in Python?
Using an index number that is out of the range of the list
You'll get the Indexerror: list index out of range error when you try and access an item using a value that is out of the index range of the list and does not exist.
This is quite common when you try to access the last item of a list, or the first one if you're using negative indexing.
Let's go back to the list we've used so far.
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
Say I want to access the last item, "Lenny", and try to do so by using the following code:
print(names[4])
#output
#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 3, in <module>
# print(names[4])
#IndexError: list index out of range
Generally, the index range of a list is 0 to n-1, with n being the total number of values in the list.
With the total values of the list above being 4, the index range is 0 to 3.
Now, let's try to access an item using negative indexing.
Say I want to access the first item in the list, "Kelly", by using negative indexing.
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
print(names[-5])
#output
#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 3, in <module>
# print(names[-5])
#IndexError: list index out of range
When using negative indexing, the index range of a list is -1 to -n, where -n the total number of items contained in the list.
With the total number of items in the list being 4, the index range is -1 to -4.
Using the wrong value in the range() function in a Python for loop
You'll get the Indexerror: list index out of range error when iterating through a list and trying to access an item that doesn't exist.
One common instance where this can occur is when you use the wrong integer in Python's range() function.
The range() function typically takes in one integer number, which indicates where the counting will stop.
For example, range(5) indicates that the counting will start from 0 and end at 4.
So, by default, the counting starts at position 0, is incremented by 1 each time, and the number is up to – but not including – the position where the counting will stop.
Let's take the following example:
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
for name in range(5):
print(names[name])
#output
#Kelly
#Nelly
#Jimmy
#Lenny
#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 7, in <module>
# print(names[name])
#IndexError: list index out of range
Here, the list names has four values.
I wanted to loop through the list and print out each value.
When I used range(5) I was telling the Python interpreter to print the values that are at the positions 0 to 4.
However, there is no item in position 4.
You can see this by first printing out the number of the position and then the value at that position.
#0
#Kelly
#1
#Nelly
#2
#Jimmy
#3
#Lenny
#4
#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 8, in <module>
# print(names[name])
#IndexError: list index out of range
You see that at position 0 is "Kelly", at position 1 is "Nelly", at position 2 is "Jimmy" and at position 3 is "Lenny".
When it comes to position four, which was specified with range(5) which indicates positions of 0 to 4, there is nothing to print out and therefore the interpreter throws an error.
One way to fix this is to lower the integer in range():
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
for name in range(4):
print(name)
print(names[name])
#output
#0
#Kelly
#1
#Nelly
#2
#Jimmy
#3
#Lenny
Another way to fix this when using a for loop is to pass the length of the list as an argument to the range() function. You do this by using the len() built-in Python function, as shown in an earlier section:
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
for name in range(len(names)):
print(names[name])
#output
#Kelly
#Nelly
#Jimmy
#Lenny
When passing len() as an argument to range(), make sure that you don't make the following mistake:
names = ["Kelly", "Nelly", "Jimmy", "Lenny"]
for name in range(len(names) + 1):
print(names[name])
After running the code, you'll again get an IndexError: list index out of range error:
#Kelly
#Nelly
#Jimmy
#Lenny
#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 4, in <module>
# print(names[name])
#IndexError: list index out of range
Conclusion
Hopefully this article gave you some insight into why the IndexError: list index out of range error occurs and some ways you can avoid it.
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Thanks for reading and happy coding!