To-do list

Table of Contents

Planning a Simple To-Do List Program

A to-do list app is a classic beginner project: it’s small, useful, and lets you practice user input, lists, conditions, loops, and files (if you want to save tasks).

In this chapter you’ll:

Design a text-based to-do list
Build a basic version that runs while the program is open
Add features: marking done, deleting tasks
Optionally, save and load tasks from a file

You can type and run all examples in a single .py file.

Designing the Program (Before Coding)

A basic to-do list program usually needs to:

Store tasks (somewhere in memory, usually a list)
Show tasks to the user
Add new tasks
Mark tasks as done (optional but very useful)
Delete tasks (optional)
Keep running until the user chooses to exit

A simple text menu is enough:

1 – Show tasks
2 – Add task
3 – Mark task as done
4 – Delete task
5 – Quit

You don’t need all of these at once. Start small and add features step by step.

Step 1: Representing Tasks

We need a way to store each task. For a very first version, each task can just be a string in a list:

A list named tasks
Each item is a piece of text: "Buy milk", "Study Python"

Later, we can add a “done/not done” status.

Basic structure:

tasks = []
# add a task
tasks.append("Buy milk")
# show all tasks
for task in tasks:
    print(task)

This gives you a simple in-memory list. When the program ends, the list is lost (we’ll fix that later with files).

Step 2: A Simple Menu Loop

To make a usable to-do list, we’ll use a loop that:

Prints a menu
Asks the user what they want to do
Runs the chosen option
Repeats until the user chooses to quit

tasks = []
while True:
    print("\nTo-Do List")
    print("1. Show tasks")
    print("2. Add task")
    print("3. Quit")
    choice = input("Choose an option (1-3): ")
    if choice == "1":
        print("Showing tasks...")
    elif choice == "2":
        print("Adding a task...")
    elif choice == "3":
        print("Goodbye!")
        break
    else:
        print("Invalid choice, please try again.")

This is the “skeleton” of the program. Now we fill in the parts for showing and adding tasks.

Step 3: Showing and Adding Tasks

Showing all tasks with numbers

It’s helpful to number tasks so the user can refer to them later:

def show_tasks(tasks):
    if not tasks:  # list is empty
        print("No tasks yet!")
        return
    print("\nYour tasks:")
    for index, task in enumerate(tasks, start=1):
        print(f"{index}. {task}")

enumerate(tasks, start=1) gives you both the position and the task text.
We start at 1 because that’s more natural for users than starting at 0.

Adding a new task

A simple function for adding a task:

def add_task(tasks):
    new_task = input("Enter a new task: ").strip()
    if new_task == "":
        print("Task cannot be empty.")
        return
    tasks.append(new_task)
    print("Task added.")

Now connect these functions to the menu loop:

tasks = []
def show_tasks(tasks):
    if not tasks:
        print("No tasks yet!")
        return
    print("\nYour tasks:")
    for index, task in enumerate(tasks, start=1):
        print(f"{index}. {task}")
def add_task(tasks):
    new_task = input("Enter a new task: ").strip()
    if new_task == "":
        print("Task cannot be empty.")
        return
    tasks.append(new_task)
    print("Task added.")
while True:
    print("\nTo-Do List")
    print("1. Show tasks")
    print("2. Add task")
    print("3. Quit")
    choice = input("Choose an option (1-3): ")
    if choice == "1":
        show_tasks(tasks)
    elif choice == "2":
        add_task(tasks)
    elif choice == "3":
        print("Goodbye!")
        break
    else:
        print("Invalid choice, please try again.")

This is already a working to-do list (without saving or marking done).

Step 4: Marking Tasks as Done

We want to track whether a task is complete. Two simple options:

Store tasks as strings, and decorate them
Example: "Buy milk [DONE]"
Store tasks as dictionaries
Example: {"text": "Buy milk", "done": False}

The second option is cleaner. Let’s switch to that.

Changing task structure

Instead of:

tasks.append(new_task)  # a string

Use:

tasks.append({"text": new_task, "done": False})

Update show_tasks to display [ ] or [x]:

def show_tasks(tasks):
    if not tasks:
        print("No tasks yet!")
        return
    print("\nYour tasks:")
    for index, task in enumerate(tasks, start=1):
        status = "[x]" if task["done"] else "[ ]"
        print(f"{index}. {status} {task['text']}")

Now add a function to mark one task as done:

def mark_task_done(tasks):
    if not tasks:
        print("No tasks to mark as done.")
        return
    show_tasks(tasks)
    choice = input("Enter the number of the task to mark as done: ")
    if not choice.isdigit():
        print("Please enter a valid number.")
        return
    index = int(choice) - 1  # convert to list index
    if index < 0 or index >= len(tasks):
        print("Task number out of range.")
        return
    tasks[index]["done"] = True
    print(f"Marked task {index + 1} as done.")

Add this option to the menu:

while True:
    print("\nTo-Do List")
    print("1. Show tasks")
    print("2. Add task")
    print("3. Mark task as done")
    print("4. Quit")
    choice = input("Choose an option (1-4): ")
    if choice == "1":
        show_tasks(tasks)
    elif choice == "2":
        add_task(tasks)
    elif choice == "3":
        mark_task_done(tasks)
    elif choice == "4":
        print("Goodbye!")
        break
    else:
        print("Invalid choice, please try again.")

Now tasks stay “done” as long as the program is running.

Step 5: Deleting Tasks

Deleting lets you remove tasks completely.

We can reuse the same style of input checking from marking done.

def delete_task(tasks):
    if not tasks:
        print("No tasks to delete.")
        return
    show_tasks(tasks)
    choice = input("Enter the number of the task to delete: ")
    if not choice.isdigit():
        print("Please enter a valid number.")
        return
    index = int(choice) - 1
    if index < 0 or index >= len(tasks):
        print("Task number out of range.")
        return
    removed = tasks.pop(index)
    print(f"Deleted task: {removed['text']}")

Update the menu again:

while True:
    print("\nTo-Do List")
    print("1. Show tasks")
    print("2. Add task")
    print("3. Mark task as done")
    print("4. Delete task")
    print("5. Quit")
    choice = input("Choose an option (1-5): ")
    if choice == "1":
        show_tasks(tasks)
    elif choice == "2":
        add_task(tasks)
    elif choice == "3":
        mark_task_done(tasks)
    elif choice == "4":
        delete_task(tasks)
    elif choice == "5":
        print("Goodbye!")
        break
    else:
        print("Invalid choice, please try again.")

At this point, you have a fully interactive to-do list while the program is running.

Step 6: Saving and Loading Tasks (Optional)

Right now, tasks disappear when you close the program. To keep them, we can:

Write them to a file when the user quits
Read them from the file when the program starts

We’ll use a plain text file with one task per line.

Simple text format

We need to save both the text and the done status. One simple format:

0;Buy milk → not done
1;Learn Python → done

Where:

The first part is 0 or 1 (0 = not done, 1 = done)
Then a ;
Then the task text

Saving tasks to a file

def save_tasks(tasks, filename="tasks.txt"):
    with open(filename, "w", encoding="utf-8") as file:
        for task in tasks:
            done_flag = "1" if task["done"] else "0"
            line = done_flag + ";" + task["text"] + "\n"
            file.write(line)
    print("Tasks saved.")

Loading tasks from a file

def load_tasks(filename="tasks.txt"):
    tasks = []
    try:
        with open(filename, "r", encoding="utf-8") as file:
            for line in file:
                line = line.rstrip("\n")
                if ";" not in line:
                    continue  # skip invalid lines
                done_flag, text = line.split(";", 1)
                task = {
                    "text": text,
                    "done": (done_flag == "1")
                }
                tasks.append(task)
    except FileNotFoundError:
        # No saved tasks yet
        tasks = []
    return tasks

Now use these in your main program:

def show_tasks(tasks):
    if not tasks:
        print("No tasks yet!")
        return
    print("\nYour tasks:")
    for index, task in enumerate(tasks, start=1):
        status = "[x]" if task["done"] else "[ ]"
        print(f"{index}. {status} {task['text']}")
def add_task(tasks):
    new_task = input("Enter a new task: ").strip()
    if new_task == "":
        print("Task cannot be empty.")
        return
    tasks.append({"text": new_task, "done": False})
    print("Task added.")
def mark_task_done(tasks):
    if not tasks:
        print("No tasks to mark as done.")
        return
    show_tasks(tasks)
    choice = input("Enter the number of the task to mark as done: ")
    if not choice.isdigit():
        print("Please enter a valid number.")
        return
    index = int(choice) - 1
    if index < 0 or index >= len(tasks):
        print("Task number out of range.")
        return
    tasks[index]["done"] = True
    print(f"Marked task {index + 1} as done.")
def delete_task(tasks):
    if not tasks:
        print("No tasks to delete.")
        return
    show_tasks(tasks)
    choice = input("Enter the number of the task to delete: ")
    if not choice.isdigit():
        print("Please enter a valid number.")
        return
    index = int(choice) - 1
    if index < 0 or index >= len(tasks):
        print("Task number out of range.")
        return
    removed = tasks.pop(index)
    print(f"Deleted task: {removed['text']}")
def save_tasks(tasks, filename="tasks.txt"):
    with open(filename, "w", encoding="utf-8") as file:
        for task in tasks:
            done_flag = "1" if task["done"] else "0"
            line = done_flag + ";" + task["text"] + "\n"
            file.write(line)
    print("Tasks saved.")
def load_tasks(filename="tasks.txt"):
    tasks = []
    try:
        with open(filename, "r", encoding="utf-8") as file:
            for line in file:
                line = line.rstrip("\n")
                if ";" not in line:
                    continue
                done_flag, text = line.split(";", 1)
                task = {
                    "text": text,
                    "done": (done_flag == "1")
                }
                tasks.append(task)
    except FileNotFoundError:
        tasks = []
    return tasks
# Main program
tasks = load_tasks()
while True:
    print("\nTo-Do List")
    print("1. Show tasks")
    print("2. Add task")
    print("3. Mark task as done")
    print("4. Delete task")
    print("5. Save and quit")
    choice = input("Choose an option (1-5): ")
    if choice == "1":
        show_tasks(tasks)
    elif choice == "2":
        add_task(tasks)
    elif choice == "3":
        mark_task_done(tasks)
    elif choice == "4":
        delete_task(tasks)
    elif choice == "5":
        save_tasks(tasks)
        print("Goodbye!")
        break
    else:
        print("Invalid choice, please try again.")

Now your to-do list remembers tasks between runs.

Step 7: Possible Extensions

Once the basic version works, you can extend it in many ways. Some ideas:

Edit a task: change its text
Show only incomplete tasks
Show statistics: number of tasks, number done
Due dates or priorities: add more fields to each task dictionary
Sort tasks: for example, incomplete first
Confirm deletes: ask “Are you sure?” before deleting

Each new feature is a chance to practice functions, input handling, and data structures.

Summary

By building this to-do list, you practiced:

Using a loop to create a menu
Storing structured data in lists of dictionaries
Reading and validating user input
Modifying items in a list (marking done, deleting)
Saving and loading data with files

This project combines many basic Python skills into a single, useful program you can keep improving as you learn more.

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