What is a List?

So far, you have worked with individual variables that each store a single value. But what if you need to store a collection of related items -- like a list of student names, a series of test scores, or a set of temperatures? That is exactly what Python lists are designed for.

Lists are one of the four built-in collection data types in Python (alongside tuples, sets, and dictionaries). They are by far the most commonly used.

Creating Lists

You create a list by placing items inside square brackets [], separated by commas.

Basic List Creation

# A list of strings
fruits = ["apple", "banana", "cherry"]
print(fruits)

# A list of numbers
scores = [95, 87, 92, 78, 100]
print(scores)

# A list with mixed data types
mixed = ["Alice", 25, True, 3.14]
print(mixed)

Empty Lists

You can create an empty list and add items to it later.

# Two ways to create an empty list
empty1 = []
empty2 = list()

print(empty1)
print(empty2)
print(type(empty1))

Using the list() Constructor

You can also create a list from other sequences using the list() function.

# Convert a string to a list of characters
letters = list("hello")
print(letters)

# Convert a range to a list
numbers = list(range(1, 6))
print(numbers)

Accessing Elements with Indexing

Every item in a list has a position number called an index. In Python, indexing starts at 0, not 1.

fruits = ["apple", "banana", "cherry", "date", "elderberry"]
#          0          1          2         3          4

print(fruits[0])   # First element
print(fruits[1])   # Second element
print(fruits[4])   # Fifth element (last)

Common Mistake: Index Out of Range

If you try to access an index that does not exist, Python raises an IndexError.

fruits = ["apple", "banana", "cherry"]

# This will cause an error!
print(fruits[3])   # IndexError: list index out of range

Remember: for a list with 3 items, valid indexes are 0, 1, and 2 (not 3).

Negative Indexing

Python allows you to use negative indexes to access elements from the end of the list. The last element is at index -1, the second-to-last is at -2, and so on.

fruits = ["apple", "banana", "cherry", "date", "elderberry"]
#          -5         -4         -3        -2         -1

print(fruits[-1])   # Last element
print(fruits[-2])   # Second to last
print(fruits[-5])   # First element

# Get the most recent score, no matter how many there are
scores = [88, 92, 75, 95, 89]
latest_score = scores[-1]
print(f"Latest score: {latest_score}")

Finding the Length of a List

Use the built-in len() function to find out how many items are in a list.

fruits = ["apple", "banana", "cherry"]
print(len(fruits))

# Useful for getting the last valid index
last_index = len(fruits) - 1
print(f"Last index: {last_index}")
print(f"Last item: {fruits[last_index]}")

# Empty list has length 0
empty = []
print(len(empty))

shopping = ["milk", "eggs", "bread"]

if len(shopping) > 0:
    print(f"You have {len(shopping)} items to buy.")
else:
    print("Your shopping list is empty!")

Checking Membership with in

Use the in keyword to check whether an item exists in a list. It returns True or False.

fruits = ["apple", "banana", "cherry"]

print("banana" in fruits)
print("grape" in fruits)

# Using 'not in'
print("grape" not in fruits)

Using in with Conditionals

allowed_users = ["alice", "bob", "charlie"]
username = "bob"

if username in allowed_users:
    print(f"Welcome, {username}!")
else:
    print("Access denied.")