Lesson 31: Lists - Python Course

Introduction to Lists

Lists are one of Python's most versatile data structures. They allow you to store multiple items in a single variable, making it easy to work with collections of data. Lists are ordered, changeable (mutable), and can contain duplicate values.

Lists are perfect for storing sequences of items like shopping lists, scores, names, or any collection of related data that you need to access, modify, or iterate over.

Creating Lists

Lists are created using square brackets [] with items separated by commas. Lists can contain any data type, including mixed types:

creating_lists.py
# Empty list
empty_list = []

# List of strings
fruits = ["apple", "banana", "cherry"]

# List of numbers
numbers = [1, 2, 3, 4, 5]

# Mixed data types
mixed = ["Alice", 25, 5.6, True]

# List with duplicate values
scores = [85, 90, 85, 95]

print(fruits)  # ['apple', 'banana', 'cherry']
print(numbers)  # [1, 2, 3, 4, 5]
print(mixed)   # ['Alice', 25, 5.6, True]

Accessing List Items

You can access list items by their index (position). Python uses zero-based indexing, meaning the first item is at index 0:

accessing_items.py
# Access items by index
fruits = ["apple", "banana", "cherry"]

print(fruits[0])  # Output: apple (first item)
print(fruits[1])  # Output: banana (second item)
print(fruits[2])  # Output: cherry (third item)

# Negative indexing (from the end)
print(fruits[-1])  # Output: cherry (last item)
print(fruits[-2])  # Output: banana (second to last)

# List length
print(len(fruits))  # Output: 3

Modifying Lists

Lists are mutable, meaning you can change their contents after creation:

modifying_lists.py
# Change an item
fruits = ["apple", "banana", "cherry"]
fruits[1] = "orange"
print(fruits)  # ['apple', 'orange', 'cherry']

# Add items (we'll learn methods in next lesson)
fruits.append("grape")
print(fruits)  # ['apple', 'orange', 'cherry', 'grape']

# Lists can grow and shrink
numbers = [1, 2, 3]
numbers.append(4)
print(numbers)  # [1, 2, 3, 4]

List Slicing

Slicing allows you to get a subset of a list by specifying a range of indices:

slicing.py
# List slicing
numbers = [0, 1, 2, 3, 4, 5]

print(numbers[1:4])   # [1, 2, 3] (indices 1 to 3)
print(numbers[:3])    # [0, 1, 2] (from start to index 2)
print(numbers[3:])    # [3, 4, 5] (from index 3 to end)
print(numbers[::2])   # [0, 2, 4] (every 2nd item)
print(numbers[-3:])  # [3, 4, 5] (last 3 items)

# Slicing creates a new list
subset = numbers[1:4]
print(subset)  # [1, 2, 3]

Iterating Over Lists

You can loop through lists using for loops:

iterating.py
# Loop through list items
fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

# Loop with index using enumerate
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Loop through numbers
numbers = [10, 20, 30]
for num in numbers:
    print(num * 2)  # 20, 40, 60

Checking if Item Exists

Use the in keyword to check if an item exists in a list:

checking_items.py
# Check if item exists
fruits = ["apple", "banana", "cherry"]

if "apple" in fruits:
    print("Apple is in the list")

if "orange" not in fruits:
    print("Orange is not in the list")

# Use in conditional statements
item = "banana"
if item in fruits:
    print(f"{item} found!")

Best Practices

✅ List Usage Tips

• Use lists for ordered collections of items

• Lists can contain any data type, including other lists

• Remember: indices start at 0, not 1

• Use negative indices to access from the end

• Lists are mutable - you can modify them

💡 Important Notes

• Lists use square brackets []

• Items are separated by commas

• Lists preserve order (items stay in the order you add them)

• Lists can contain duplicate values

Lists