Creating and Accessing Lists

A list is an ordered collection of items enclosed in square brackets []. Lists can hold any data type and are one of the most versatile data structures in Python.

# Creating lists
fruits = ["apple", "banana", "cherry", "date"]
scores = [85, 92, 78, 95, 88]
mixed = ["hello", 42, True, 3.14]
empty = []

# Accessing elements (indexing starts at 0)
print(fruits[0])     # apple
print(fruits[-1])    # date (last item)
print(fruits[1:3])   # ['banana', 'cherry'] (slicing)

# Length of a list
print(len(fruits))   # 4

Output: apple | date | ['banana', 'cherry'] | 4

Modifying Lists

Lists are mutable, meaning you can change them after creation. You can add, remove, and modify elements using built-in methods.

colours = ["red", "green", "blue"]

# Adding items
colours.append("yellow")       # Add to end
colours.insert(1, "orange")    # Insert at index 1
print(colours)  # ['red', 'orange', 'green', 'blue', 'yellow']

# Removing items
colours.remove("green")        # Remove by value
popped = colours.pop()         # Remove and return last item
print(popped)   # yellow
print(colours)  # ['red', 'orange', 'blue']

# Modifying items
colours[0] = "purple"          # Change first item
print(colours)  # ['purple', 'orange', 'blue']

# Sorting
numbers = [3, 1, 4, 1, 5, 9]
numbers.sort()
print(numbers)  # [1, 1, 3, 4, 5, 9]

List Comprehensions

A list comprehension is a concise way to create a new list by transforming or filtering items from an existing list. It uses the syntax [expression for item in list].

# Create a list of squares
squares = [x ** 2 for x in range(1, 6)]
print(squares)  # [1, 4, 9, 16, 25]

# Filter: only even numbers
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = [n for n in numbers if n % 2 == 0]
print(evens)    # [2, 4, 6, 8, 10]

# Transform: uppercase all strings
names = ["alice", "bob", "charlie"]
upper_names = [name.upper() for name in names]
print(upper_names)  # ['ALICE', 'BOB', 'CHARLIE']

Key Vocabulary

List

An ordered, mutable collection of items enclosed in square brackets [].

Index

The position of an element in a list, starting from 0 for the first element.

Mutable

Can be changed after creation. Items can be added, removed, or modified.

List Comprehension

A concise one-line syntax for creating lists: [expr for item in list].

Worked Examples

1

Calculate the average of a list of scores

scores = [85, 92, 78, 95, 88]

total = sum(scores)
count = len(scores)
average = total / count

print(f"Scores: {scores}")
print(f"Total: {total}")
print(f"Average: {average}")

Step 1: sum() adds all elements, len() counts the elements.

Output: Scores: [85, 92, 78, 95, 88] | Total: 438 | Average: 87.6

2

Build a shopping list with add and remove

shopping = ["milk", "bread", "eggs"]
print(f"Initial: {shopping}")

# Add items
shopping.append("butter")
shopping.insert(0, "cereal")
print(f"After adding: {shopping}")

# Remove an item
shopping.remove("bread")
print(f"After removing bread: {shopping}")

# Check if item exists
print(f"Is 'milk' in list? {'milk' in shopping}")

Step 1: append() adds to the end; insert(0, ...) adds to the beginning.

Step 2: Use in to check membership. Returns True or False.

3

Use list comprehension to filter passing scores

all_scores = [45, 82, 67, 91, 38, 75, 53, 88]

# Filter scores >= 50 (passing)
passing = [s for s in all_scores if s >= 50]
failing = [s for s in all_scores if s < 50]

print(f"Passing: {passing}")
print(f"Failing: {failing}")
print(f"Pass rate: {len(passing)}/{len(all_scores)}")

Step 1: The list comprehension filters items based on the if condition.

Output: Passing: [82, 67, 91, 75, 53, 88] | Failing: [45, 38] | Pass rate: 6/8

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

What does ["a", "b", "c"][1] return?

"a" "b" "c" ["b"]

Question 2

Which method adds an item to the end of a list?

.insert() .add() .append() .push()

Question 3

What does [x * 2 for x in [1, 2, 3]] produce?

[1, 2, 3, 1, 2, 3] [1, 4, 9] [3, 6, 9] [2, 4, 6]

Question 4

What does len([10, 20, 30, 40]) return?

4 3 40 100

Question 5

What does [1, 2, 3][-1] return?

1 3 2 Error

Python Lists