[python 일반] [Python] How to Fix Python’s “List Index Out of Range” Error in For Loops

The List Index Out of Range error often occurs when working with lists and for loops. You see, in Python, when you attempt to access an element using an index that lies outside the valid index range of the list, you're essentially telling the program to fetch something that isn't there, resulting in this common error.

It's the interpreter's way of signaling that there's a misalignment in your expectations of the list's size and the actual indices present.

Let’s take a closer look at common ways a for loop can cause List Index Out of Range and how to either avoid it completely or gracefully handle this error when it crops up.

What causes the “List Index Out of Range” error?

As Python uses zero-based indexing, when you try to access an element at an index less than 0 or greater than or equal to the list’s length, Python tells you via this error that the specified index is out of the permissible bounds of the list's length. Here are some common scenarios when this error occurs:

Incorrect loop indexing

If an index used in a loop across a range of indices is greater than the list's length, the error IndexError: list Index Out of Range occurs.

Example: In the below code, the loop runs four times, and on the fourth iteration, my_list[3] is accessed, which doesn’t exist, raising the error.

my_list = [1, 2, 3]
for i in range(4):
    print(my_list[i])

Output:

1
2
3
Traceback (most recent call last):
    File "C:\Users\name\AppData\Local\Programs\Python\Python311\check.py", line 3, in <module>
print (my_list[i])
IndexError: list index out of range

Changing the list inside the loop

If the list is updated within the loop like removing elements it can cause the loop to go past the updated list length raising the error.

Example: In the below code, the second iteration removes the element, reducing the list’s length to 2, but still, the loop proceeds one more time, raising the error.

my_list = [10, 20, 30]
for i in range(len(my_list)):
    if i == 1:
        my_list.pop(0)  # Removing an element from the list
    print(my_list[i])

Output:

10
30
Traceback (most recent call last):
    File "C:\Users\name\AppData\Local\Programs\Python\Python311\check.py", line 5, in <module>
print (my_list[i])
IndexError: list index out of range

Incorrect list length calculation

If you mention the wrong condition inside the for loop, you’ll encounter this error.

Example: In the below code, my_list[3] will be accessed, which doesn’t exist, raising the error.

my_list = [10, 20, 30]
for i in range(len(my_list) + 1):
    print(my_list[i])

Output:

10
20
30
Traceback (most recent call last):
    File "C:\Users\name\AppData\Local\Programs\Python\Python311\check.py", line 5, in <module>
print (my_list[i])
IndexError: list index out of range

How to resolve the “List Index Out of Range” error in for loops

Below are some ways to tackle the List Index Out of Range error when working with for loops.

Use enumerate()

You can make use of the enumerate() function to iterate over both the indices and elements of the list simultaneously. This makes sure that you stay within the bounds of the list.

경축! 아무것도 안하여 에스천사게임즈가 새로운 모습으로 재오픈 하였습니다.
어린이용이며, 설치가 필요없는 브라우저 게임입니다.
https://s1004games.com

Example:

my_list = [5, 120, 18]
for i, element in enumerate(my_list):
    print(f"Index {i}: {element}")

Output:

Index 0: 5
Index 1: 120
Index 2: 18

Precalculate the list’s length

Before iterating over a list it’s a good practice to pre-calculate the length of the list.

Example:

my_list = [9, 18, 30]
list_length = len(my_list)
for i in range(list_length):
    print(my_list[i])

Output:

9
18
30

Handle list modification

Whenever modifying the list inside the loop, it’s better to use a copy of the list or a different data structure to avoid altering the loop’s behavior.

Example:

my_list = [50, 100, 1785]
for i in range(len(my_list)):
    if i == 1:
        copied_list = my_list.copy()  # Create a copy to avoid modification
        copied_list.pop(0)  # Removing an element from the list
    print(my_list[i])

Output:

50
100
1785

Use try-catch blocks

You can wrap your index access inside a try-catch block to catch the exception and handle it gracefully.

Example:

try:
    my_list = [20, 40, 60]
    for i in range(4):
        try:
            print(my_list[i])
        except IndexError as e:
            print("Error:", e)
            print("Index", i, "is out of range")
except Exception as e:
    print("An error has occurred:", e)

Output:

20
40
60
Error: list index out of range
Index 3 is out of range

[출처] https://rollbar.com/blog/how-to-fix-python-list-index-out-of-range-error-in-for-loops/