3.10 Lists in Python

A list is one of the most versatile and commonly used data structures in Python. It is an ordered, mutable collection of items that can store elements of different data types.

Creating a List

Lists are defined using square brackets [] with elements separated by commas.

# Empty list
empty_list = []

# List of integers
numbers = [10, 20, 30, 40]

# List of mixed data types
mixed = [1, 'hello', 3.14, True]

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

Key Properties of Lists

Indexing

Python uses zero-based indexing — the first element is at index 0.

fruits = ['apple', 'banana', 'cherry']
print(fruits[0])   # Output: apple
print(fruits[1])   # Output: banana
print(fruits[2])   # Output: cherry

Negative Indexing

Negative indices count from the end of the list. Index -1 is the last element.

fruits = ['apple', 'banana', 'cherry']
print(fruits[-1])  # Output: cherry
print(fruits[-2])  # Output: banana

Mutability — Modifying a List

Because lists are mutable, you can change any element by assigning a new value to its index.

L = [1, 2, 3]
L[1] = 5
print(L)  # Output: [1, 5, 3]

List Slicing

Slicing extracts a sub-list using the syntax:

list[start:stop:step]

• start — index to begin (inclusive)
• stop — index to end (exclusive)
• step — interval between elements (default is 1)

L = [0, 1, 2, 3, 4, 5]
print(L[1:4])    # Output: [1, 2, 3]
print(L[0:6:2])  # Output: [0, 2, 4]
print(L[:3])     # Output: [0, 1, 2]
print(L[3:])     # Output: [3, 4, 5]

Common List Methods

append() — Add to End

L = [1, 2, 3]
L.append(4)
print(L)  # Output: [1, 2, 3, 4]

insert() — Add at Specific Index

L = [1, 2, 3]
L.insert(1, 10)  # Insert 10 at index 1
print(L)  # Output: [1, 10, 2, 3]

pop() — Remove by Index

L = [1, 2, 3, 4]
L.pop(2)         # Removes element at index 2
print(L)  # Output: [1, 2, 4]

remove() — Remove by Value

L = [1, 2, 3, 2]
L.remove(2)      # Removes first occurrence of 2
print(L)  # Output: [1, 3, 2]

len() — Length of List

A = [10, 20, 30, 40]
print(len(A))  # Output: 4

sort() — Sort the List

L = [3, 1, 4, 1, 5]
L.sort()
print(L)  # Output: [1, 1, 3, 4, 5]

Iterating Over a List

Lists are commonly used with loops to process each element — this directly supports translating repetition algorithms into Python.

fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

Using range() with Lists

numbers = [10, 20, 30, 40, 50]
for i in range(len(numbers)):
    print(f"Index {i}: {numbers[i]}")

Building a List with a Loop

A common algorithmic pattern is to build a list by appending elements inside a loop:

squares = []
for i in range(1, 6):
    squares.append(i ** 2)
print(squares)  # Output: [1, 4, 9, 16, 25]