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)

Inserting and Removing Elements

append(val): Appends an item to the end of the list.
insert(idx, val): Inserts an item at a specific index.
pop(): Removes and returns the last item in the list.
pop(idx): Removes and returns the item at a specific index.

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

fruits.append("date")         # ["apple", "banana", "cherry", "date"]
fruits.insert(1, "mango")     # ["apple", "mango", "banana", "cherry", "date"]

last = fruits.pop()           # Removes and returns "date"
second = fruits.pop(1)        # Removes and returns "mango"
print(fruits)                 # ["apple", "banana", "cherry"]

Sorting: `sort()` vs `sorted()`

list.sort(): Sorts the list in-place (mutates the original list; returns None).
sorted(list): A built-in function that returns a new sorted list, leaving the original list unchanged.

numbers = [5, 2, 8, 1]

# 1. sorted() function (creates a new list)
new_sorted = sorted(numbers)
print("Original:", numbers)   # [5, 2, 8, 1] (unchanged)
print("Sorted Copy:", new_sorted) # [1, 2, 5, 8]

# 2. sort() method (modifies in place)
numbers.sort()
print("Original (modified):", numbers) # [1, 2, 5, 8]

Mathematical Operations: `sum()`, `min()`, `max()`

For lists containing numeric values, Python provides helpful built-in mathematical functions:

scores = [78, 92, 85, 60]

print(sum(scores))  # 315 (calculates total sum)
print(min(scores))  # 60 (finds minimum value)
print(max(scores))  # 92 (finds maximum value)

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

Practice & Exercises

To reinforce what you’ve learned in this section (Lists, indexing, slicing, and operations), practice with these interactive notebooks:

Follow-Along Practice

Practice initializing lists, accessing elements, slicing, and list manipulation.💻 VS Code | 🚀 Colab | 📥 Download

Practice Exercises

Test your knowledge with hands-on exercises on shopping lists and list slicing.💻 VS Code | 🚀 Colab | 📥 Download

What’s next?

Now let’s learn about Tuples - perfect for storing ordered data that should never change!

Tuples

Learn about immutable collections

Getting started

Data types

Working with data

Control flow

Data structures

Packing & Unpacking

What are lists?

Creating lists

Accessing items

Changing lists

List methods

Inserting and Removing Elements

Sorting: `sort()` vs `sorted()`

Mathematical Operations: `sum()`, `min()`, `max()`

Checking lists

Common mistakes

Practice & Exercises

Follow-Along Practice

Practice Exercises

What’s next?

Tuples

​What are lists?

​Creating lists

​Accessing items

​Changing lists

​List methods

​Inserting and Removing Elements

​Sorting: sort() vs sorted()

​Mathematical Operations: sum(), min(), max()

​Checking lists

​Common mistakes

​Practice & Exercises

Follow-Along Practice

Practice Exercises

​What’s next?

Tuples

What are lists?

Creating lists

Accessing items

Changing lists

List methods

Inserting and Removing Elements

Sorting: `sort()` vs `sorted()`

Mathematical Operations: `sum()`, `min()`, `max()`

Checking lists

Common mistakes

Practice & Exercises

What’s next?