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.

Record parameters, scikit-learn version, feature schema, class ordering, baseline metrics, and resource needs. If serializing, load artifacts only from trusted sources and verify their version compatibility.

from sklearn.ensemble import RandomForestClassifier

X = [[1, 0], [2, 1], [3, 0], [4, 2], [5, 1], [6, 3], [7, 2], [8, 4]]
y = [0, 0, 0, 1, 1, 1, 1, 1]

forest = RandomForestClassifier(
    n_estimators=200, max_depth=3, oob_score=True,
    random_state=7, n_jobs=-1,
)
forest.fit(X, y)
prediction = forest.predict([[5, 2]])
probabilities = forest.predict_proba([[5, 2]])
print("predicted class:", prediction[0])
print("probability shape:", probabilities.shape)
print("probabilities sum to one:", probabilities[0].sum().round(6) == 1)
print("oob in range:", 0 <= forest.oob_score_ <= 1)

Expected evidence:

predicted class: 1
probability shape: (1, 2)
probabilities sum to one: True
oob in range: True

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. n_estimators sets the number of independently grown trees whose results are combined.
2. max_depth limits each member's complexity.
3. a fixed random_state makes bootstrap and feature sampling reproducible.
4. predict_proba reports vote fractions by class; out-of-bag scoring uses rows omitted from each tree's bootstrap sample.

These lines form one chain: rows of [hours, practice_tests] and binary readiness labels becomes majority-vote classes, class probabilities, and evaluation evidence from rows not used by individual trees. Change only one input first. When several values change together, you cannot tell which change caused the new behavior.

• First failure: very small datasets can produce unreliable out-of-bag scores and warnings because too few independent votes exist. Re-run the smallest command that proves the repair.
• Second failure: feature order drift still changes predictions even though forests accept the numeric shape. Preserve the failing input as a test when it represents a realistic mistake.
• Misleading success: more trees reduce Monte Carlo variation but cannot correct wrong labels, leakage, or a biased sample. 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 small tabular classifier must remain stable when one noisy training row changes.

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 reproducible forest classifier with held-out predictions, vote probabilities, and an out-of-bag estimate.

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.