Python List Index: Get and Change Items

Get an Item by Its Index

Every item in a list has a position number, called its index. To get one item, write the list's name and the index in square brackets: friends[0].

Think of the floors in a building where the ground floor is floor 0. The first floor you reach is 0, the next one is 1, and so on.

1234friends = ["Sunny", "Rocky", "Lucky"]
print(friends[0])
print(friends[1])
print(friends[2])

The rules

• The first item has index 0, not 1.
• The last item has index len(list) - 1. Three items means indexes 0, 1 and 2.
• The index is a whole number. friends[1.0] gives a TypeError.

Why start at 0? Read the index as "how many steps from the start". The first item is 0 steps away, the second is 1 step away.

Use an item like any value

prices[0] is a value, just like a variable. You can print it, store it, or calculate with it:

123prices = [40, 25, 60]
total = prices[0] + prices[2]
print(f"Total: {total}")

Common beginner doubt: which one is the second item?

friends[1] is the second item, "Rocky". Every beginner reaches for friends[1] when they want the first item. If your program shows the wrong item, check whether you started counting at 0.

The index counts from 0: list[0] is the first item, list[1] the second.

Negative Index: Count from the End

A negative index counts from the end of the list. -1 is the last item, -2 is the one before it, and so on.

Think of a queue at a shop. You can say "the last person in the queue" without counting everyone from the front.

1234friends = ["Sunny", "Rocky", "Lucky"]
print(friends[-1])
print(friends[-2])
print(friends[-3])

Every item has two indexes, one from the front and one from the back:

The rules

• -1 is always the last item, whatever the length of the list.
• There is no "minus zero". -0 is the same as 0, the first item.
• The first item is at -len(list): -3 in a list of three.

Why it's handy

You often want the newest item, and the newest item is usually at the end. A negative index gets it without knowing the length:

123scores = [12, 45, 33, 27, 50]
print("Latest score:", scores[-1])
print("Same item:", scores[len(scores) - 1])

Good: scores[-1]. Short and clear.

Avoid: scores[len(scores) - 1]. It works, but it's longer and easy to get wrong. Forget the - 1, and you get an error (next lesson).

-1 is the last item and -2 the one before it. Negative indexes count from the end.

IndexError: list index out of range

Ask for an index that the list doesn't have, and Python stops with an IndexError.

Think of a lift in a three-floor building. Press the button for floor 5, and nothing can take you there: that floor doesn't exist.

12friends = ["Sunny", "Rocky", "Lucky"]
print(friends[3])

Don't be scared of this error! It's just saying: "this index is outside the list". The list has three items, so the indexes go from 0 to 2. There is no index 3.

The rules

• A list of length n has indexes 0 to n - 1.
• From the back, it has -1 to -n.
• Anything outside that range is an IndexError.

The most common cause

Using len() as the index. len(friends) is 3, but the last index is 2:

12friends = ["Sunny", "Rocky", "Lucky"]
print(friends[len(friends)])

Every error is a clue! When you see IndexError, print the list and its length, then compare them with the index you used.

Check the index first

When the index comes from the user, you can't know it in advance. Check it with an if from chapter 8 before you use it:

123456friends = ["Sunny", "Rocky", "Lucky"]
n = int(input())
if 0 <= n < len(friends):
    print(friends[n])
else:
    print("No friend at that index")

Try 0, 2 and 3 in the Input box. Only 0, 1 and 2 pass the check.

Valid indexes go from 0 to len(list) - 1. Anything else is an IndexError.

Change an Item in a List

You can change an item in a list: put the index on the left of =, and the new value on the right. Lists that can be changed like this are called mutable (a fancy word for "changeable").

Think of a timetable on a whiteboard. Rub out one lesson and write another in its place. The rest of the week stays the same.

123menu = ["tea", "coffee", "juice"]
menu[1] = "lassi"
print(menu)

The rules

• list[index] = value replaces the item at that index. It's reassignment from chapter 4, for one item.
• The other items don't move.
• Negative indexes work too: menu[-1] = "water" changes the last item.
• The index must already exist. This can change items, but it can't add new ones.

Change an item using its own value

Like score = score + 5 in chapter 4, an item can get a new value worked out from its old one. Even += from chapter 5 works:

1234scores = [10, 20, 30]
scores[0] = scores[0] + 5
scores[-1] += 1
print(scores)

Avoid: an index that isn't there

12menu = ["tea", "coffee"]
menu[2] = "juice"

The list has indexes 0 and 1. Index 2 doesn't exist yet, so Python can't put anything there. To add a new item at the end, use append() from topic 4.

Change an item with list[index] = value. The index must already exist.

Practice: Indexing

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 72.Python Program to Print the First and Last Item of a ListEasySolve
2. 73.Python Program to Find the Day Name from Its NumberMediumSolve