Changing List Elements

You can also change an element of a list. Write the list's name and the index in square brackets, as when you read the element, and assign it a new value with =, just as you would assign a value to a variable:

resolutions = ["Keep a diary", "Take a falconry course", "Learn to juggle"]
resolutions[1] = "Take a painting course"
print(resolutions)

This prints:

['Keep a diary', 'Take a painting course', 'Learn to juggle']

The element at index 1 now has a new value, and the old one is gone. The other elements stay as they were. An element of a list works like a variable, so += works too: scores[0] += 5 adds 5 to the first element of the list scores.

You can only change an element that exists. Assigning to an index past the end of the list doesn't add a new element:

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

The program ends with this error:

Traceback (most recent call last):
  File "<exec>", line 2, in <module>
IndexError: list assignment index out of range

This time, the error message talks about a list assignment index: the index you assigned a value to. You'll learn how to add an element to the end of a list in a moment.