Lists — collections of values

Every variable you have used so far holds exactly one value. That works fine for a single temperature or a person's name, but most programs deal with collections: a list of students, a set of prices, the words in a sentence. Creating a separate variable for each item is unworkable.

Python's list stores an ordered sequence of values in a single variable. You can have as many items as you need, access any one by its position, add new items, and loop over the whole collection with a for loop.

Creating a list and indexing

A list is written with square brackets, items separated by commas:

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

Access an individual item by its index — its position, counting from zero:

print(fruits[0])   # apple
print(fruits[1])   # banana
print(fruits[2])   # cherry

Python counts from 0. The first item is index 0, the second is index 1, and so on. This is a universal convention shared by nearly every programming language.

Negative indices count from the end:

print(fruits[-1])  # cherry  (last item)
print(fruits[-2])  # banana  (second to last)

Length with len()

len() tells you how many items a list contains:

print(len(fruits))   # 3

Adding and removing items

append adds an item to the end:

fruits.append("date")
print(fruits)   # ['apple', 'banana', 'cherry', 'date']

pop removes and returns the last item (or the item at an index you specify):

last = fruits.pop()
print(last)     # date
print(fruits)   # ['apple', 'banana', 'cherry']

Iterating over a list

A for loop works directly on lists — you do not need indices:

scores = [88, 92, 75, 100, 60]

for score in scores:
    if score >= 90:
        print(score, "— excellent!")
    else:
        print(score)

When you do need the index and the value together, use enumerate:

for i, fruit in enumerate(fruits):
    print(i, fruit)

Exercise

Where to go next

Next: the First steps lab — three exercises that combine everything you have learned so far into real mini-programs.