So far one variable has held one value. A list holds many, in order, in a single variable.

fruits = ["apple", "banana", "cherry"]
print(fruits)
print(len(fruits))

Square brackets create a list, commas separate the items, and len() tells you how many there are. A list can hold any type — even a mix, though in practice you usually keep one kind of thing per list.

numbers = [4, 8, 15, 16]
mixed = ["Ada", 36, True]
empty = []

Indexing

Each item has a position, called its index. Python counts from 0, not 1.

fruits = ["apple", "banana", "cherry"]
print(fruits[0])
print(fruits[1])
print(fruits[2])

So the first item is [0] and the last item of a three-item list is [2]. Asking for fruits[3] raises an IndexError — there is no fourth slot.

Negative indexes

Counting backwards from the end is common enough that Python builds it in. -1 is the last item, -2 the second-to-last.

fruits = ["apple", "banana", "cherry"]
print(fruits[-1])
print(fruits[-2])

This saves you from writing fruits[len(fruits) - 1].

Changing an item

Unlike strings, lists are mutable — you can change them in place.

fruits = ["apple", "banana", "cherry"]
fruits[1] = "blueberry"
print(fruits)

Slicing

A slice pulls out a section, using [start:stop]. As with range(), the stop is not included.

numbers = [0, 1, 2, 3, 4, 5]
print(numbers[1:4])
print(numbers[:3])
print(numbers[3:])
print(numbers[-2:])

Leave out the start to mean "from the beginning," and leave out the stop to mean "to the end." A slice always gives you back a new list; the original is untouched.

Looping over a list

for walks a list the same way it walked a string in Module 2.

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

Try it yourself

Print the first item, the last item, and the middle two items of the list below.

Checkpoint

• Lists hold many ordered values: [1, 2, 3].
• Indexes start at 0; -1 is the last item.
• len() gives the length; an out-of-range index raises IndexError.
• Slices [start:stop] exclude the stop and return a new list.
• Lists are mutable — you can reassign an item by index.