Organizing a CLI App

beginner12 min readLesson 53 of 169

Layers: controller, storage, models — and rules that keep them sane.

Everything you have learned converges into an application shape:

todo/
  app.py        # menu loop, argparse — the "controller"
  storage.py    # load_tasks() / save_tasks() — JSON file I/O
  models.py     # the task dict shape + validation helpers
  tests.py      # asserts over add/complete/delete logic

Rules that keep it sane:

Functions take data in and return data out — add_task(tasks, text), never a function that reaches into globals.
All file access lives in storage. Swap JSON for a database later; nothing else changes.
Validate at the edges. Bad input is rejected once, at the boundary, with a clear message.
The menu loop is the dumbest part of the program. Logic belongs below.

The capstone asks you to invent this structure yourself. This module is the rehearsal.

Now practice

Mini Build: Persistent Task Manager CoreLoad, mutate, save — the storage half of every CLI app.3 challenges · · ~40 min