Build a Typed, Tested CLI Tool

• ✓Parse messy real-world text into typed, immutable domain objects
• ✓Process a file too large for memory with a generator pipeline, and measure the difference
• ✓Define a plugin boundary with a Protocol, and explain why it is not an ABC
• ✓Validate untrusted input once, at the boundary, and account for every rejected line instead of crashing or dropping it
• ✓Ship a packaged CLI with an exit-code contract, separate stdout and stderr, and machine-readable output
• ✓Write tests that check a known ground truth, and prove they catch real bugs
• ✓Gate a codebase with ruff, mypy --strict and pytest in CI, and run the tool against everything that should go wrong

Build the kind of small, typed, tested command-line tool a working Python engineer ships — and the package every later drill in the track turns into an API, a database-backed service and a deployed product.