Lists

What are lists?

Lists are Python’s most versatile data structure. They’re like containers that can hold multiple items in a specific order. Think of a list like:

A shopping list (milk, eggs, bread)
A to-do list (tasks in order)
A playlist (songs in sequence)

Creating lists

# Empty list
my_list = []

# List with items
fruits = ["apple", "banana", "orange"]
numbers = [1, 2, 3, 4, 5]
mixed = ["hello", 42, True, 3.14]  # Different types OK!

Lists use square brackets [] and items are separated by commas. You can mix different data types in the same list!

Accessing items

Lists are indexed starting at 0:

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

# Get items
print(fruits[0])    # "apple" (first item)
print(fruits[1])    # "banana"
print(fruits[-1])   # "orange" (last item)
print(fruits[-2])   # "banana" (second to last)

# Slicing
print(fruits[0:2])  # ["apple", "banana"]
print(fruits[1:])   # ["banana", "orange"]

Changing lists

Lists are mutable - you can change them:

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

# Change an item
fruits[0] = "mango"
print(fruits)  # ["mango", "banana", "orange"]

# Add items
fruits.append("grape")      # Add to end
fruits.insert(1, "kiwi")    # Insert at position

# Remove items
fruits.remove("banana")     # Remove by value
last = fruits.pop()        # Remove and return last
del fruits[0]              # Remove by index

List methods

numbers = [3, 1, 4, 1, 5, 9]

# Information
print(len(numbers))         # 6 (length)
print(numbers.count(1))     # 2 (count occurrences)
print(numbers.index(4))     # 2 (find position)

# Sorting
numbers.sort()              # Sort in place
print(numbers)              # [1, 1, 3, 4, 5, 9]

numbers.reverse()           # Reverse order
print(numbers)              # [9, 5, 4, 3, 1, 1]

# Copy
new_list = numbers.copy()   # Create a copy

Checking lists

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

# Check if item exists
if "apple" in fruits:
    print("Found apple!")

# Check if list is empty
if fruits:
    print("List has items")
else:
    print("List is empty")

Common mistakes

Index out of range

# Wrong
fruits = ["apple", "banana"]
print(fruits[2])  # IndexError!

# Right - check length first
if len(fruits) > 2:
    print(fruits[2])

Modifying while looping

# Wrong - changes list size during loop
numbers = [1, 2, 3, 4]
for num in numbers:
    if num == 2:
        numbers.remove(num)  # Dangerous!

# Right - use list comprehension
numbers = [num for num in numbers if num != 2]

Shallow copy issues

# Wrong - both variables point to same list
list1 = [1, 2, 3]
list2 = list1
list2.append(4)
print(list1)  # [1, 2, 3, 4] - changed!

# Right - make a copy
list1 = [1, 2, 3]
list2 = list1.copy()
list2.append(4)
print(list1)  # [1, 2, 3] - unchanged

What’s next?

Now let’s learn about dictionaries - perfect for storing related information!

Dictionaries

Key-value pairs

Getting started

Data types

Working with data

Control flow

Data structures

What are lists?

Creating lists

Accessing items

Changing lists

List methods

Checking lists

Common mistakes

What’s next?

Dictionaries

​What are lists?

​Creating lists

​Accessing items

​Changing lists

​List methods

​Checking lists

​Common mistakes

​What’s next?

Dictionaries

What are lists?

Creating lists

Accessing items

Changing lists

List methods

Checking lists

Common mistakes

What’s next?