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23. Lists

1. Introduction​

A list is an ordered, changeable collection of items.
Lists are one of the most commonly used data structures in Python because they are flexible and easy to use.

2. Creating Lists​

Lists are defined using square brackets []:

fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", 3.14, True]

3. Accessing List Elements​

Use indexes (starting at 0):

fruits = ["apple", "banana", "cherry"]
print(fruits[0])   # apple
print(fruits[-1])  # cherry (last element)

4. Modifying Lists​

Lists are mutable, meaning you can change them:

fruits[1] = "blueberry"
print(fruits)  # ['apple', 'blueberry', 'cherry']

5. Adding Items​

  • Append to the end:
fruits.append("orange")
  • Insert at a position:
fruits.insert(1, "mango")

6. Removing Items​

fruits.remove("apple")   # remove by value
print(fruits)

fruits.pop(1)            # remove by index
print(fruits)

last = fruits.pop()      # remove last item
print(last)

fruits.clear()           # remove all items
print(fruits)  # []

7. List Slicing​

numbers = [10, 20, 30, 40, 50]
print(numbers[1:3])  # [20, 30]
print(numbers[:3])   # [10, 20, 30]
print(numbers[2:])   # [30, 40, 50]
print(numbers[::-1]) # reversed list

8. Checking Membership​

fruits = ["apple", "banana"]
print("apple" in fruits)   # True
print("mango" not in fruits) # True

9. Iterating Over a List​

for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

10. Useful List Methods​

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

print(len(numbers))      # 5
print(numbers.count(1))  # 2
print(numbers.index(4))  # 2
numbers.sort()
print(numbers)           # [1, 1, 3, 4, 5]
numbers.reverse()
print(numbers)           # [5, 4, 3, 1, 1]

11. Nested Lists​

Lists can contain other lists:

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print(matrix[1][2])  # 6

12. Copying Lists​

Be careful when copying lists:

a = [1, 2, 3]
b = a
b[0] = 99
print(a)  # [99, 2, 3] (changes reflected!)

# Correct way:
c = a.copy()
c[0] = 42
print(a)  # [99, 2, 3]
print(c)  # [42, 2, 3]

13. When to Use Lists​

  • When you need an ordered collection.
  • When you need to frequently modify elements.
  • For storing sequences of data like names, scores, or items.

14. Next Steps​

✅ You now understand how to create, access, modify, and use lists.
In the next chapter, we’ll explore tuples, which are similar to lists but immutable.