Lists Collections

Welcome back. You probably know how to repeat actions with loops (for example, prompting for a password until it’s correct). But when you have a collection of items — daily goals, shopping items, or any group of values — lists let you walk through those items one at a time and perform operations on each. This article teaches Python lists: how to create them, access and modify items, loop through them, and use common list methods. By the end of this lesson you’ll be able to:

create lists and access items by index,
add and remove items,
loop through lists with for loops,
and use lists to store and process collections of data.

Create a new script called lists.py and follow along.

What is a list?

A list is a mutable sequence that can store multiple items. Use square brackets to define a list:

numbers = [1, 2, 3, 4, 5]

Lists can hold mixed types:

mixed = ["Hello", 42, True, 3.14]

This flexibility (storing different types together) distinguishes Python lists from lower-level arrays in some other languages. Lists are general-purpose, can be changed in-place, and are easy to iterate over.

Accessing items by index

Consider a shopping list:

shopping = ["Eggs", "Bread", "Milk", "Butter"]

Each item has an index (position). Indexes start at 0:

shopping[0] -> "Eggs"
shopping[1] -> "Bread"
shopping[2] -> "Milk"
shopping[3] -> "Butter"

Example:

shopping = ["Eggs", "Bread", "Milk", "Butter"]

print(shopping[0])
print(shopping[1])
print(shopping[3])

Expected output:

Eggs
Bread
Butter

Negative indexing

You can count from the end using negative indexes. -1 is the last item, -2 the second-to-last, and so on:

print(shopping[-1])  # Butter
print(shopping[-2])  # Milk

To print "Milk" you can use either:

print(shopping[2])
# or
print(shopping[-2])

Modifying items

Assign a new value to an index to overwrite an item:

shopping = ["Eggs", "Bread", "Milk", "Butter"]

print(shopping)           # ['Eggs', 'Bread', 'Milk', 'Butter']
shopping[1] = "Naan"      # replace "Bread"
print(shopping)           # ['Eggs', 'Naan', 'Milk', 'Butter']
shopping[-1] = "Ghee"     # replace "Butter" using negative index
print(shopping)           # ['Eggs', 'Naan', 'Milk', 'Ghee']

Adding and removing items

Use append() to add to the end, remove() to delete by value, and pop() to remove by index and return the removed item.

shopping = ['Eggs', 'Naan', 'Milk', 'Ghee']
print(shopping)

shopping.append("Mango")
print(shopping)  # ['Eggs', 'Naan', 'Milk', 'Ghee', 'Mango']

# Remove by value (deletes the first occurrence)
shopping.remove("Milk")
print(shopping)  # ['Eggs', 'Naan', 'Ghee', 'Mango']

# Or pop by index and capture the removed item
shopping = ['Eggs', 'Naan', 'Milk', 'Ghee', 'Mango']
item = shopping.pop(2)   # removes index 2 ("Milk")
print("You removed:", item)
print(shopping)          # ['Eggs', 'Naan', 'Ghee', 'Mango']

Note: remove() takes a value and deletes the first matching element; pop() takes an index and returns the removed element.

Common list operations

len(my_list) — number of items in the list.
item in my_list — membership test (case-sensitive for strings).
Loop with for item in my_list: to process each element.

Example:

shopping = ['Eggs', 'Naan', 'Ghee', 'Mango']

total = len(shopping)
print("Your basket has", total, "items")

if "Ghee" in shopping:
    print("Ghee is in the basket")
else:
    print("Ghee is not in the basket")

Be aware that membership checks are case-sensitive:

# This will be False because "ghee" (lowercase) != "Ghee"
"ghee" in shopping  # False

To perform a case-insensitive membership test, normalize strings first:

"ghee".lower() in [item.lower() for item in shopping]

A code editor window (showing Python code to collect goals and a terminal) fills most of the screen. A man in a black t‑shirt stands on the right, gesturing toward the code.

Quick reference — list methods and operations

Operation	Description	Example
Create	Define a new list	`items = [1, 2, 3]`
Access	Get item by index	`items[0]`
Negative index	Access from end	`items[-1]`
Replace	Assign to an index	`items[1] = 42`
Append	Add to end	`items.append("x")`
Remove by value	Delete first matching value	`items.remove("x")`
Pop by index	Remove & return element	`removed = items.pop(0)`
Length	Number of elements	`len(items)`
Membership	Check if present	`"x" in items`
Iterate	Loop over items	`for item in items:`

For full details, see the official Python lists documentation: https://docs.python.org/3/tutorial/datastructures.html

Putting it together: a simple goals tracker

Let’s build a small program that:

collects goals from the user until they type done,
shows the total number of goals,
prompts the user to mark which goals are complete,
displays completed and remaining counts and a final message.

Save this as lists.py:

goals = []

print("Enter your goals then 'done' to finish:")
while True:
    goal = input("Goal: ")
    if goal.lower() == "done":
        break
    goals.append(goal)

print("\nTotal goals:", len(goals))
print("Mark completed goals:")

completed = 0
for goal in goals:
    status = input(goal + " - done? (y/n): ")
    if status.lower().startswith("y"):
        completed += 1

print("\nTotal goals:", len(goals))
print("Completed:", completed)
print("Remaining:", len(goals) - completed)

if completed == len(goals):
    print("You nailed all your goals today.")
else:
    print("Keep going - you're making progress!")

Example run (user input shown after prompts):

Enter your goals then 'done' to finish:
Goal: start
Goal: finish
Goal: done

Total goals: 2
Mark completed goals:
start - done? (y/n): y
finish - done? (y/n): n

Total goals: 2
Completed: 1
Remaining: 1
Keep going - you're making progress!

Recap

Lists are defined with square brackets and can store multiple items, including mixed types.
Access list items by zero-based index or by negative index to count from the right.
Modify lists with index assignment, append(), remove(), and pop().
Use len() for the count and in for membership tests.
Iterate with for item in my_list: to process each element.
Normalize strings (for example, with .lower()) when checking user input or doing case-insensitive comparisons.

For further reading:

Python official tutorial — Data Structures: https://docs.python.org/3/tutorial/datastructures.html
Practical Python examples and exercises — search for “Python list examples”

Tip: When checking user input (like y/n) or looking for items in a list, normalize strings with .lower() to avoid case-sensitivity issues.

Introduction

Building Computer Programs

Final Project

What is a list?

Accessing items by index

Negative indexing

Modifying items

Adding and removing items

Common list operations

Quick reference — list methods and operations

Putting it together: a simple goals tracker

Recap

Watch Video

Practice Lab

​What is a list?

​Accessing items by index

​Negative indexing

​Modifying items

​Adding and removing items

​Common list operations

​Quick reference — list methods and operations

​Putting it together: a simple goals tracker

​Recap

Watch Video

Practice Lab

What is a list?

Accessing items by index

Negative indexing

Modifying items

Adding and removing items

Common list operations

Quick reference — list methods and operations

Putting it together: a simple goals tracker

Recap