Python To-Do Application

In the last lesson, Weather Application, you fetched live data from the internet. Now we come back to building a tool you would actually keep open every day: a to-do list. You add tasks, mark them done, and remove them, and the app remembers everything between runs. This project ties together the lists, dictionaries, and file handling you have practiced, into a clean little app. By the end you will have a working task manager you can really use.

🤔 What makes a good to-do app?

A to-do list sounds simple, but think about what it really needs to do. It is more than just a list of strings.

A useful to-do app should:

• Add a new task.
• Show all tasks, clearly marking which are done.
• Mark a task as done when you finish it.
• Remove a task you no longer need.
• Remember everything after you close it.

Notice each task has two parts: the text of the task, and whether it is done yet. So a plain string is not enough. We will store each task as a small dictionary with a title and a done flag. That extra flag is what lets us tick things off.

🧱 Step 1: The task data

Each task is a dictionary with the task text and a done value that is True or False. All the tasks live in a list. We also reuse our save and load helpers.

This is the data shape and the storage functions:

import json

# One task: {"title": "Buy milk", "done": False}
# All tasks live in a list.

def save(tasks):
    with open("tasks.json", "w") as f:
        json.dump(tasks, f)

def load():
    try:
        with open("tasks.json", "r") as f:
            return json.load(f)
    except FileNotFoundError:
        return []

So each task carries its own done status. A new task starts with "done": False, and we flip it to True when it is finished. The save and load functions are the same pattern you have used before, with an empty list as the fresh start.

🛠️ Step 2: The task functions

Now the actions: add a task, show the tasks, mark one done, and delete one. We refer to each task by its number in the list, as the user sees it.

These functions handle the four actions:

def add_task(tasks, title):
    tasks.append({"title": title, "done": False})

def show_tasks(tasks):
    if not tasks:
        print("No tasks yet.")
        return
    for i, task in enumerate(tasks, start=1):
        mark = "x" if task["done"] else " "
        print(f"{i}. [{mark}] {task['title']}")

def mark_done(tasks, number):
    if 1 <= number <= len(tasks):
        tasks[number - 1]["done"] = True
        print("Marked done.")
    else:
        print("No task with that number.")

def delete_task(tasks, number):
    if 1 <= number <= len(tasks):
        removed = tasks.pop(number - 1)
        print(f"Removed: {removed['title']}")
    else:
        print("No task with that number.")

Let’s read the tricky parts:

• enumerate(tasks, start=1) gives both a number (starting at 1) and each task as we loop, so we can show “1.”, “2.” and so on. Humans count from 1, even though lists start at 0.
• The mark is "x" if the task is done, or a space if not, giving a nice [x] or [ ] checkbox look.
• For mark_done and delete_task, the user gives a number starting at 1, but the list starts at 0. So we use number - 1 to reach the right item. The check 1 <= number <= len(tasks) makes sure the number is valid first.

🖥️ Step 3: The complete program

Here is the full to-do app with a menu loop, saving after each change. Save it as todo.py and run it.

import json

def save(tasks):
    with open("tasks.json", "w") as f:
        json.dump(tasks, f)

def load():
    try:
        with open("tasks.json", "r") as f:
            return json.load(f)
    except FileNotFoundError:
        return []

def add_task(tasks, title):
    tasks.append({"title": title, "done": False})

def show_tasks(tasks):
    if not tasks:
        print("No tasks yet.")
        return
    for i, task in enumerate(tasks, start=1):
        mark = "x" if task["done"] else " "
        print(f"{i}. [{mark}] {task['title']}")

def mark_done(tasks, number):
    if 1 <= number <= len(tasks):
        tasks[number - 1]["done"] = True
        print("Marked done.")
    else:
        print("No task with that number.")

def delete_task(tasks, number):
    if 1 <= number <= len(tasks):
        removed = tasks.pop(number - 1)
        print(f"Removed: {removed['title']}")
    else:
        print("No task with that number.")

def get_number(prompt):
    try:
        return int(input(prompt))
    except ValueError:
        return -1

def main():
    tasks = load()
    print("To-Do List")

    while True:
        print("\n1) Add  2) Show  3) Mark done  4) Delete  5) Quit")
        choice = input("Choose an option: ")

        if choice == "1":
            title = input("Task: ")
            add_task(tasks, title)
            save(tasks)
            print("Added!")
        elif choice == "2":
            show_tasks(tasks)
        elif choice == "3":
            show_tasks(tasks)
            mark_done(tasks, get_number("Number to mark done: "))
            save(tasks)
        elif choice == "4":
            show_tasks(tasks)
            delete_task(tasks, get_number("Number to delete: "))
            save(tasks)
        elif choice == "5":
            print("Goodbye!")
            break
        else:
            print("Pick 1 to 5.")

main()

Here is a sample run:

To-Do List

1) Add  2) Show  3) Mark done  4) Delete  5) Quit
Choose an option: 1
Task: Buy milk
Added!

1) Add  2) Show  3) Mark done  4) Delete  5) Quit
Choose an option: 3
1. [ ] Buy milk
Number to mark done: 1
Marked done.

1) Add  2) Show  3) Mark done  4) Delete  5) Quit
Choose an option: 2
1. [x] Buy milk

1) Add  2) Show  3) Mark done  4) Delete  5) Quit
Choose an option: 5
Goodbye!

There it is: a real to-do app. You can add tasks, tick them off with a clear [x], remove them, and it all stays saved between runs. This is a tool you could genuinely use, built entirely from skills you now have.

⚠️ Common Mistakes

A few traps in this project:

• Mixing up the user number and the list position. The user counts from 1, the list from 0. Forgetting number - 1 either crashes or changes the wrong task.
• Not checking the number is valid. If the user types a number with no matching task, reaching that position raises an IndexError. The range check prevents it.
• Storing tasks as plain strings. Without a done flag, you cannot mark anything finished. A dictionary per task gives each one a status.

✅ Best Practices

Habits this project teaches:

• Store each item as a dictionary when it has more than one property, like a title and a done flag.
• Show numbered lists from 1 with enumerate(..., start=1), and convert back with number - 1 to reach the list.
• Always validate the chosen number before using it, so a bad number never crashes the app.
• Save after every change, and load at the start, so the list survives between runs.

🧩 What You’ve Learned

✅ You built a to-do app that adds, shows, completes, and deletes tasks, and remembers them between runs.

✅ Each task is a dictionary with a title and a done flag, so you can tick items off.

✅ enumerate(tasks, start=1) shows a list numbered from 1, the way people expect.

✅ User numbers start at 1 but lists start at 0, so you convert with number - 1, after checking the number is valid.

✅ Saving to JSON after each change keeps the list safe between runs.

🚀 What’s Next?

You built a polished everyday tool. Next we focus on the data side: an API data fetcher that pulls information from an online service and saves it neatly to a file.

API Data Fetcher