Lists

Sometimes one variable is not enough.

You may want to store:

• Many names
• Many numbers
• Many products
• Many tasks

Creating many separate variables becomes hard to manage.

Python gives us lists to solve this problem.

What Is a List?

A list is a collection of values stored in one variable.

Lists use square brackets [].

names = ["Tom", "Sara", "Mike"]
print(names)

Output

['Tom', 'Sara', 'Mike']

Why Lists Matter

Lists help you:

• Store many values together
• Keep data organized
• Add new items
• Remove items
• Change items
• Search data
• Loop through items easily

Creating Lists

numbers = [10, 20, 30]
colors = ["Red", "Blue", "Green"]
mixed = ["Book", 10, True]

A list can store text, numbers, booleans, or mixed values.

Reading Items with Indexes

Each item has a position called an index.

The first item starts at 0.

fruits = ["Apple", "Banana", "Orange"]

print(fruits[0])
print(fruits[1])
print(fruits[2])

Output

Apple
Banana
Orange

Negative Indexes

Negative indexes count from the end.

• -1 last item
• -2 second last item

print(fruits[-1])
print(fruits[-2])

Output

Orange
Banana

Updating Items

Lists can be changed.

fruits = ["Apple", "Banana", "Orange"]
fruits[1] = "Grape"

print(fruits)

Output

['Apple', 'Grape', 'Orange']

Adding Items with append()

Adds one item to the end.

fruits = ["Apple", "Banana"]
fruits.append("Orange")

print(fruits)

Output

['Apple', 'Banana', 'Orange']

Adding Items with insert()

Adds an item at a chosen position.

fruits = ["Apple", "Orange"]
fruits.insert(1, "Banana")

print(fruits)

Output

['Apple', 'Banana', 'Orange']

Adding Many Items with extend()

Adds multiple items.

fruits = ["Apple"]
fruits.extend(["Banana", "Orange"])

print(fruits)

Output

['Apple', 'Banana', 'Orange']

Removing with remove()

Removes by value.

fruits = ["Apple", "Banana", "Orange"]
fruits.remove("Banana")

print(fruits)

Output

['Apple', 'Orange']

Removing with pop()

Removes by index.

fruits = ["Apple", "Banana", "Orange"]
fruits.pop(1)

print(fruits)

Output

['Apple', 'Orange']

Clearing a List

Removes all items.

items = ["Pen", "Book"]
items.clear()

print(items)

Output

[]

Finding Item Position

Use index().

fruits = ["Apple", "Banana", "Orange"]
print(fruits.index("Banana"))

Output

1

Counting Items

Use count().

numbers = [1, 2, 2, 3]
print(numbers.count(2))

Output

2

Sorting Items

Use sort().

numbers = [3, 1, 2]
numbers.sort()

print(numbers)

Output

[1, 2, 3]

Reversing Items

Use reverse().

numbers = [1, 2, 3]
numbers.reverse()

print(numbers)

Output

[3, 2, 1]

What Is Slicing?

Slicing means taking part of a list.

Basic format:

list[start:stop:step]

• start = where to begin
• stop = where to stop (not included)
• step = how many positions to move

Use this list:

fruits = ["Apple", "Banana", "Orange", "Grape", "Mango"]

Slicing with Start and Stop

print(fruits[1:4])

Output

['Banana', 'Orange', 'Grape']

From Beginning

print(fruits[:3])

Output

['Apple', 'Banana', 'Orange']

To the End

print(fruits[2:])

Output

['Orange', 'Grape', 'Mango']

Copy a List

print(fruits[:])

Output

['Apple', 'Banana', 'Orange', 'Grape', 'Mango']

Using Step

Take every second item.

print(fruits[::2])

Output

['Apple', 'Orange', 'Mango']

Start, Stop, Step Together

print(fruits[1:5:2])

Output

['Banana', 'Grape']

Reverse with Slicing

print(fruits[::-1])

Output

['Mango', 'Grape', 'Orange', 'Banana', 'Apple']

Looping Through a List

names = ["Tom", "Sara", "Mike"]

for name in names:
    print(name)

Output

Tom
Sara
Mike

List Length

Use len().

items = ["Pen", "Book", "Bag"]
print(len(items))

Output

3

Code Along

cities = ["New York", "Toronto", "Chicago"]
cities.append("Vancouver")

for city in cities:
    print(city)

Output

New York
Toronto
Chicago
Vancouver

Mini Challenge

Build a shopping list.

Steps:

• Create a list with:
  Milk, Bread, Eggs
• Add Juice
• Remove Bread
• Print the first two items using slicing
• Print the final list

Expected output:

['Milk', 'Eggs']
['Milk', 'Eggs', 'Juice']

Real World Use Case

Programs use lists for shopping carts, contact lists, tasks, products, and storing many records.

Quiz

1. What is a list?
2. What index number is the first item?
3. What does append() do?
4. What does slicing do?

Assignment

Create a list of 5 favorite movies. Add one movie, remove one movie, then print the first three items using slicing.

Summary

You learned how to create lists, read items, update data, remove data, use methods, slice lists, and loop through list values.