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Module 020: Milestone Project: To-Do List CLI App

Phase: 2. Control Flow & Data | Estimated time: 3 hours | Milestone Project: Yes

Prerequisites

Learning Objectives

  • Build a complete CLI application from scratch
  • Design a menu-driven interface with a while loop
  • Manage data using lists and related structures
  • Implement CRUD operations (Create, Read, Update, Delete)
  • Apply all concepts from Modules 001-019

Why This Matters

This milestone project brings together everything you've learned so far: variables, conditionals, loops, lists, string formatting, and user input. Building a real tool — even a simple one — is the best way to solidify your skills. You'll also learn how to structure a program, handle edge cases, and think like a developer.

Concept Explanation

Project Overview

You'll build a To-Do List CLI App that runs in the terminal. The app will let users:

  • View all tasks
  • Add a new task
  • Mark a task as complete
  • Delete a task
  • Quit the app

Data Design

Since we haven't learned dictionaries yet (Module 023), we'll use two parallel lists:

tasks = []           # list of task descriptions (strings)
completed = []       # list of booleans tracking completion

When the user adds "Buy milk", we append to both lists:

  • tasks → ["Buy milk"]
  • completed → [False]

Menu Structure

while True:
    print("\n===== TO-DO LIST =====")
    print("1. View tasks")
    print("2. Add task")
    print("3. Mark complete")
    print("4. Delete task")
    print("5. Quit")
    choice = input("Choose (1-5): ")
    # handle choice...

Displaying Tasks

def show_tasks(tasks, completed):
    if not tasks:
        print("No tasks yet!")
        return
    for i, task in enumerate(tasks):
        status = "[✓]" if completed[i] else "[ ]"
        print(f"{i + 1}. {status} {task}")

Adding a Task

def add_task(tasks, completed):
    description = input("Enter task description: ")
    if not description:
        print("Task cannot be empty!")
        return
    tasks.append(description)
    completed.append(False)
    print(f"Added: {description}")

Marking a Task Complete

def complete_task(tasks, completed):
    show_tasks(tasks, completed)
    if not tasks:
        return
    try:
        index = int(input("Task number to mark complete: ")) - 1
        if 0 <= index < len(tasks):
            if completed[index]:
                print("That task is already complete!")
            else:
                completed[index] = True
                print(f"Marked complete: {tasks[index]}")
        else:
            print("Invalid task number.")
    except ValueError:
        print("Please enter a number.")

Deleting a Task

def delete_task(tasks, completed):
    show_tasks(tasks, completed)
    if not tasks:
        return
    try:
        index = int(input("Task number to delete: ")) - 1
        if 0 <= index < len(tasks):
            removed = tasks.pop(index)
            completed.pop(index)
            print(f"Deleted: {removed}")
        else:
            print("Invalid task number.")
    except ValueError:
        print("Please enter a number.")

Common Pitfalls

  1. Not handling empty lists — If the user chooses "Mark complete" with no tasks, the program should show a friendly message, not crash.

  2. Off-by-one in index display — Show tasks starting at 1 for the user, but subtract 1 to access the list (0-indexed).

  3. Not validating input — The user might enter a letter when asked for a number. Use try/except or .isdigit().

  4. Allowing empty task descriptions — A task with no description is meaningless. Check if not description before adding.

  5. Forgetting to update both parallel lists — When deleting a task, you must pop() from both tasks and completed.

Hands-On Walkthrough

Your starter code is in project/starter_code.py. It has a main() function with the menu loop and placeholder comments. Your job is to fill in each section.

Here's the full solution structure:

def main():
    tasks = []
    completed = []

    while True:
        print("\n===== TO-DO LIST =====")
        print("1. View tasks")
        print("2. Add task")
        print("3. Mark complete")
        print("4. Delete task")
        print("5. Quit")

        choice = input("Choose (1-5): ")

        if choice == "1":
            if not tasks:
                print("\nNo tasks yet!")
            else:
                print()
                for i in range(len(tasks)):
                    status = "[✓]" if completed[i] else "[ ]"
                    print(f"{i + 1}. {status} {tasks[i]}")

        elif choice == "2":
            desc = input("Enter task description: ")
            if desc:
                tasks.append(desc)
                completed.append(False)
                print(f"Added: {desc}")
            else:
                print("Task cannot be empty!")

        elif choice == "3":
            if not tasks:
                print("\nNo tasks yet!")
                continue
            for i in range(len(tasks)):
                status = "[✓]" if completed[i] else "[ ]"
                print(f"{i + 1}. {status} {tasks[i]}")
            try:
                idx = int(input("Task number to mark complete: ")) - 1
                if 0 <= idx < len(tasks):
                    if completed[idx]:
                        print("Already complete!")
                    else:
                        completed[idx] = True
                        print(f"Completed: {tasks[idx]}")
                else:
                    print("Invalid number.")
            except ValueError:
                print("Enter a valid number.")

        elif choice == "4":
            if not tasks:
                print("\nNo tasks yet!")
                continue
            for i in range(len(tasks)):
                status = "[✓]" if completed[i] else "[ ]"
                print(f"{i + 1}. {status} {tasks[i]}")
            try:
                idx = int(input("Task number to delete: ")) - 1
                if 0 <= idx < len(tasks):
                    removed = tasks.pop(idx)
                    completed.pop(idx)
                    print(f"Deleted: {removed}")
                else:
                    print("Invalid number.")
            except ValueError:
                print("Enter a valid number.")

        elif choice == "5":
            print("Goodbye!")
            break

        else:
            print("Invalid choice. Please enter 1-5.")

if __name__ == "__main__":
    main()

Key Takeaways

  • A menu-driven program uses a while True loop with break to exit
  • Parallel lists are a simple way to store related data without dictionaries
  • Always validate user input — both the format (number vs text) and the range
  • Display indices starting from 1 for readability; convert internally to 0-based
  • Empty collections ([], "") are falsy — use if not tasks: to check
  • try/except ValueError handles non-numeric input gracefully
  • Keep your code organized: consider breaking it into functions

Next Steps

  • Check your solution against project/solution/todo.py
  • Try extending the app: add due dates, priorities, or categories
  • Next module: Module 021: Tuples — Learn about dictionaries for key-value data

Further Reading