Standard-library CLI: argparse, json, csv (todo app capstone)

argparse builds real command-line interfaces: positional arguments (required, order-based), optional flags (--title, -t), and subcommands (todo add, todo list, todo done) via add_subparsers. This is the difference between a script and a tool someone else can run without reading the source.

import argparse

parser = argparse.ArgumentParser(prog="todo")
sub = parser.add_subparsers(dest="command", required=True)

add_p = sub.add_parser("add")
add_p.add_argument("title")

args = parser.parse_args()
if args.command == "add":
    print(f"adding: {args.title}")

json.dumps/json.loads persist structured data as text — the natural fit for a list of dataclass-shaped records. The csv module (DictReader/DictWriter) does the same for tabular data with named columns, which is what you already used in Lesson 3.

This lesson is a capstone: it doesn't teach much new syntax, it asks you to combine dataclasses, pathlib, exceptions, logging, and argparse from Lessons 1-4 into one real command-line tool.