How to Fix: IndexError: list assignment index out of range

This is a classic misunderstanding of how Python lists work compared to arrays in other languages. One common error is the IndexError list assignment issue, which often confuses those new to Python.

You cannot “assign” a value to a slot that doesn’t exist yet.

Problem Code:

my_list = []  # An empty list
my_list[0] = "Hello" 
# CRASH! IndexError: list assignment index out of range

Python says: “You want me to put ‘Hello’ at position 0, but position 0 doesn’t exist! The list has length 0.”

The Fix: .append()

To add new items to a list, you must use the .append() method. It automatically creates the new slot at the end of the list.

my_list = []
my_list.append("Hello") # Correct!
print(my_list[0]) # Now this works, output: Hello

When Assignment IS Okay

You can only assign to an index if it already exists.

my_list = ["Old Value", "Item 2"]
# This is fine because index 0 already exists
my_list[0] = "New Value"