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.

Include a tiny sample input, expected output, and whether writing replaces an existing target. For large datasets, document a streaming variant using with source.open(...).

from pathlib import Path

source = Path("labels.txt")
target = Path("clean-labels.txt")
lines = source.read_text(encoding="utf-8").splitlines()
clean = list(dict.fromkeys(
    line.strip().lower() for line in lines if line.strip()
))
target.write_text("\n".join(clean) + "\n", encoding="utf-8")
print(f"read {len(lines)}; saved {len(clean)}")

Expected evidence:

read 5; saved 3

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. Path keeps path operations clearer than manual slash concatenation.
2. read_text decodes bytes as UTF-8 and splitlines removes line endings.
3. the generator normalizes values, while dict.fromkeys removes duplicates without changing first-seen order.
4. write_text replaces the target and returns a byte count; the source path is never opened for writing.

These lines form one chain: UTF-8 lines that may contain blank space, duplicate labels, and mixed capitalization becomes one normalized label per line in a new file. Change only one input first. When several values change together, you cannot tell which change caused the new behavior.

• First failure: FileNotFoundError includes the unresolved path; print source.resolve() to verify the current working directory. Re-run the smallest command that proves the repair.
• Second failure: UnicodeDecodeError means the declared encoding does not match the bytes. Preserve the failing input as a test when it represents a realistic mistake.
• Misleading success: PermissionError or a partial write needs a safe destination; critical workflows should write a temporary file and replace only after success. 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 noisy labels.txt must become a lowercase, de-duplicated clean-labels.txt without overwriting the source.

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 script that reads UTF-8 text, cleans non-empty labels, writes atomically to a separate path, and reports counts.

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.