Build a CLI To-Do App (Python)
We're going to build a to-do app this weekend. Not a toy that prints "Hello" and quits - a real one. You'll add tasks, list them, mark them done, delete them, and save the whole thing to a file so it survives between runs. By the end you'll type python todo.py add "buy milk" in a terminal and watch it work.
Here's the part that makes this fun: every code block in this project runs right here in your browser. You don't need Python installed to follow along. Click run, watch the output, change a line, run it again. When you're ready for the real thing, the last phase shows you exactly how to drop the same code onto your own machine.
What you'll build
A single Python file - todo.py - that you drive from the command line:
python todo.py add "write the report"
python todo.py list
python todo.py done 1
Tasks live in a plain JSON file next to the script. Close your terminal, come back tomorrow, your list is still there.
The stack
Nothing to install. We use three things, all built into Python:
| Piece | What it does |
|---|---|
list + dict |
hold tasks in memory while the program runs |
json module |
save and load tasks as a text file |
sys.argv |
read the command word you typed (add, list, done) |
No frameworks. No pip. No third-party packages. The standard library does all of it, and learning to reach for it first is a habit worth building early.
What you'll learn
- How to model data as a list of dictionaries - the bread and butter of Python programs.
- How to read and write files without losing your data.
- How JSON turns Python objects into text and back again.
- How a command-line tool decides what to do based on the words you type.
Each of these is a skill you'll reuse in nearly everything you write next.
How the project flows
graph LR
A[Phase 1<br/>tasks in memory] --> B[Phase 2<br/>save to file]
B --> C[Phase 3<br/>complete & delete]
C --> D[Phase 4<br/>a real CLI]
Four phases, each one a working step:
- Tasks in Memory - represent tasks and add them to a growing list.
- Saving to a File - write tasks to JSON and read them back.
- Complete, Delete, and Filter - mark done, remove, and split open from finished.
- A Real CLI - dispatch on a command word and run it as a script on your machine.
Rough time
Plan for two or three relaxed hours. Each phase is ~30–45 minutes if you run the code and tinker. There's no rush - the point is to understand each piece before you stack the next one on top.
Who this is for
You've seen a little Python - you know what a variable and a function are - and you want to build something whole instead of more disconnected exercises. That's exactly the right place to start. Let's build it.