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.

Share the commit hash and the exact verification command. A teammate should be able to inspect the patch with git show HASH, run the relevant test, and understand why the snapshot exists.

git status --short
git diff -- train.py
git add train.py
git diff --cached
git commit -m "Add held-out model evaluation"
git log -1 --oneline

Expected evidence:

M  train.py
 M README.md
[main a1b2c3d] Add held-out model evaluation
 1 file changed, 6 insertions(+)

The output may include version-specific details such as hashes, paths, fitted thresholds, or final decimal places. Compare the structural facts described here rather than copying placeholders. If the structure differs, stop and inspect the earliest unexpected line.

1. git status --short uses two columns: the left is staged and the right is unstaged.
2. git diff inspects unstaged content, while git diff --cached inspects the proposed commit.
3. git add train.py copies that file's current change into the index; it does not upload anything.
4. git commit creates local history, and git log -1 identifies the resulting snapshot.

These lines form one chain: the working-tree versions of train.py and README.md becomes a commit containing the intended train.py diff while the README edit remains unstaged. Change only one input first. When several values change together, you cannot tell which change caused the new behavior.

• First failure: fatal: not a git repository means initialize the intended folder with git init or change into the repository. Re-run the smallest command that proves the repair.
• Second failure: an accidental staged secret must be removed from the index with git restore --staged FILE and from the file before committing. Preserve the failing input as a test when it represents a realistic mistake.
• Misleading success: a commit that says nothing to commit has no staged changes; inspect both status columns instead of repeating the command. 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 an evaluation script and README are being changed together, but only the tested evaluation change should enter the first commit.

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 small repository with a focused commit named Add held-out model evaluation and a clean working tree.

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.