Add observability and tests

Turn the stable example into repeatable checks. Capture the command, input fixture, expected output, and important boundary. Tests should be fast enough to run before every change and precise enough that a failure identifies behavior rather than just saying the chapter broke.

Merge with the repository's chosen strategy only after review. Then update local main, delete the feature branch when appropriate, and verify the merged commit or squash commit is present.

git switch -c improve-evaluation
python -m pytest
git add train.py tests/test_train.py
git commit -m "Test held-out evaluation"
git push -u origin improve-evaluation
git status -sb

Expected evidence:

2 passed in 0.08s
branch 'improve-evaluation' set up to track 'origin/improve-evaluation'.

1. git switch -c creates and checks out a local feature branch.
2. pytest supplies evidence before publication; a successful push is not proof that code works.
3. git push -u origin ... sends commits and records the upstream branch.
4. the pull request is created on GitHub after the branch exists; pushing does not merge it.

These lines form one chain: local commits on improve-evaluation plus the repository's pull-request rules becomes a remote feature branch and a reviewable proposal into main. Change only one input first. When several values change together, you cannot tell which change caused the new behavior.

• First failure: remote origin already exists means inspect git remote -v instead of adding a duplicate. Re-run the smallest command that proves the repair.
• Second failure: a rejected non-fast-forward push usually means the remote branch moved; fetch and reconcile rather than force-pushing shared history. Preserve the failing input as a test when it represents a realistic mistake.
• Misleading success: a red required check must be opened and read; rerunning without a code or infrastructure reason hides the actual failure. A clean-looking final line cannot cancel contradictory intermediate evidence.

When debugging, copy the exact error text and inspect names, paths, shapes, types, and versions. Explain the cause in one sentence before changing code. That discipline prevents a guessed repair from creating a second defect.

On this page, the practical job is to turn important behavior into repeatable checks. The running case is a branch named improve-evaluation contains one reviewed change that must pass tests before it reaches main.

For the current test step, save the smallest useful evidence: the relevant command, its output, and the input that produced it. Do not use a screenshot as the only record when text can be copied and searched. Keep generated artifacts separate from source inputs so rerunning the example does not destroy the evidence it is meant to evaluate.

The deliverable for this step is a narrow pull request with a useful summary, reproducible test steps, and a green required check.

Return to your opening prediction. Mark it correct or rewrite it with the condition you missed. Then explain the difference between a successful execution and a trustworthy result for this specific example.