IndexError: list index out of range

The error message:

IndexError: list index out of range

occurs when you try to access an index in a list that does not exist. This means the index is either negative beyond range or larger than the last available index.

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1. Causes and Solutions

Cause 1: Accessing an Index That Does Not Exist

If you try to access an index that is greater than or equal to the list’s length, Python raises an IndexError.

Incorrect Code:

numbers = [10, 20, 30]
print(numbers[3])  # Error: Index 3 is out of range (valid indices: 0, 1, 2)

Solution: Ensure the index is within the list length.

if len(numbers) > 3:
    print(numbers[3])
else:
    print("Index is out of range")

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Cause 2: Using a Loop with an Incorrect Range

If you loop beyond the valid indices, you may get this error.

Incorrect Code:

numbers = [1, 2, 3]
for i in range(4):  # Error: range(4) includes index 3, which does not exist
    print(numbers[i])

Solution: Use len(numbers) to avoid out-of-range errors.

for i in range(len(numbers)):  # Safe iteration
    print(numbers[i])

Or use a direct iteration approach.

for num in numbers:
    print(num)

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Cause 3: Negative Index Beyond List Range

Negative indices refer to elements from the end, but if they exceed the list size, an error occurs.

Incorrect Code:

numbers = [10, 20, 30]
print(numbers[-4])  # Error: Only -1, -2, and -3 are valid

Solution: Ensure the negative index is within range.

if -4 >= -len(numbers):
    print(numbers[-4])
else:
    print("Negative index out of range")

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Cause 4: Modifying a List While Iterating

If you remove items while iterating with an index, the list size changes, causing an out-of-range error.

Incorrect Code:

numbers = [1, 2, 3, 4]
for i in range(len(numbers)):
    numbers.pop(i)  # Error: List size changes, causing an invalid index

Solution: Iterate over a copy or use list comprehension.

numbers = [1, 2, 3, 4]
for num in numbers[:]:  # Iterating over a copy prevents index issues
    numbers.remove(num)
print(numbers)

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Cause 5: Using an Index Without Checking List Length

Functions that return lists (like split(), findall(), etc.) may return an empty list, leading to an out-of-range error.

Incorrect Code:

words = "Hello".split(",")  # ['Hello'], no comma found
print(words[1])  # Error: Only one element exists at index 0

Solution: Check if the list has enough elements before accessing.

if len(words) > 1:
    print(words[1])
else:
    print("Not enough elements in the list")