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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)

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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!

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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"]

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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

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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

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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")

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Common mistakes

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

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

# 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]

# 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

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What’s next?

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

Dictionaries

Key-value pairs