Manage a To-Do List – Lists and Basic Data Structures

Tools You’ll Use

• Python 3 (Google Colab, Jupyter Notebook, or local interpreter)
• Code editor/IDE (VS Code, Thonny, PyCharm, or text editor)
• Web browser (for checking docs & debugging help)

Lesson Roadmap

1. Review: Why we need data structures.
2. Learn lists: what they are and how they store multiple values.
3. Build a simple to-do list manager with options to add, view, and remove tasks.
4. Practice looping through lists to display tasks.
5. Handle invalid input and empty lists.
6. Real-world tie-ins: using code to organize your life.
7. Recap & Quiz.
8. Mini challenge: extend the app with “mark as done” and sorting.

Why Lists Matter

In the calculator and guessing game, we stored only a few numbers at a time. But what if you need to track many values: shopping items, homework tasks, or quiz questions?

A list is like a container (a basket or shelf) that can hold multiple items at once. You don’t need 10 separate variables (task1, task2, …). You just make one list:

tasks = ["Buy milk", "Finish assignment", "Call friend"]

Python remembers the order and lets you add, remove, and loop through items.

👉 Intuition tip: Think of a list as a numbered shelf where each slot has a label (index).

• tasks[0] → first item: “Buy milk”
• tasks[1] → second item: “Finish assignment”

Step 1: Creating and Modifying Lists

tasks = []   # start with an empty list

# Add items
tasks.append("Read Python notes")
tasks.append("Do laundry")
print(tasks)   # ['Read Python notes', 'Do laundry']

# Remove an item
tasks.remove("Do laundry")
print(tasks)   # ['Read Python notes']

Tips:

• Use .append(item) to add new tasks.
• Use .remove(item) to delete a specific one.
• Use len(tasks) to check how many tasks are left.

Step 2: Planning the To-Do List App

The program will:

1. Show a menu with choices (Add / View / Remove / Exit).
2. Let the user pick an action.
3. Perform the action and loop back to the menu.
4. Continue until the user chooses Exit.

Step 3: Writing the Code

tasks = []

print("Welcome to your To-Do List Manager!")

while True:
    print("\nMenu:")
    print("1. Add a task")
    print("2. View tasks")
    print("3. Remove a task")
    print("4. Exit")

    choice = input("Choose an option (1-4): ")

    if choice == "1":
        task = input("Enter a new task: ")
        tasks.append(task)
        print(f"'{task}' added to your list.")

    elif choice == "2":
        if len(tasks) == 0:
            print("No tasks yet!")
        else:
            print("Your tasks:")
            for i, task in enumerate(tasks, start=1):
                print(f"{i}. {task}")

    elif choice == "3":
        if len(tasks) == 0:
            print("No tasks to remove.")
        else:
            print("Select a task number to remove:")
            for i, task in enumerate(tasks, start=1):
                print(f"{i}. {task}")
            index = int(input("Task number: ")) - 1
            if 0 <= index < len(tasks):
                removed = tasks.pop(index)
                print(f"Removed '{removed}'")
            else:
                print("Invalid number.")

    elif choice == "4":
        print("Goodbye! 👋")
        break

    else:
        print("Invalid choice. Please enter 1-4.")

Sample Run

Welcome to your To-Do List Manager!

Menu:
1. Add a task
2. View tasks
3. Remove a task
4. Exit
Choose an option (1-4): 1
Enter a new task: Finish homework
'Finish homework' added to your list.

Menu:
1. Add a task
2. View tasks
3. Remove a task
4. Exit
Choose an option (1-4): 2
Your tasks:
1. Finish homework

Common Mistakes & Fixes

• Forgetting .append() → assigning directly (tasks = task) replaces the whole list.
• Using wrong index in .pop() (remember lists start at 0).
• Removing an item that doesn’t exist causes an error. Always check with if len(tasks) > 0:.
• Infinite loop: forgetting the break when user exits.

👉 Debugging tip: Print the list after each action to check what’s inside.

Real-World Applications

This small app mirrors real productivity tools (like Trello or Google Tasks). Managing lists of items is central to:

• Task managers.
• Shopping apps.
• School grading systems (list of students, scores).

Once you master lists, you’re on your way to handling larger data structures like tables, databases, and JSON.

Recap of Key Concepts

• Lists store multiple items in one variable.
• .append() adds to a list; .remove() or .pop() removes.
• len() counts items.
• Loops (for i, task in enumerate(tasks)) let you process each item.
• Menu-driven programs use while True loops to repeat until Exit.

Review Questions

1. What does tasks.append("New Task") do?
2. Why do lists start at index 0 in Python?
3. What happens if you try to .pop() from an empty list?

Mini Challenges

1. Mark as Done: Add an option to mark a task complete (e.g., add ✅ after it).
2. Sorting: Add an option to sort tasks alphabetically with tasks.sort().
3. Save to File: Save tasks to a text file so they remain after closing the program.
4. Deadlines: Let users add a due date (e.g., store tasks as ["Finish homework - Due Monday"]).