Repair with the Test Still Watching

The red suite is already specific: one collected tmp_path test expected visible é, while the real UTF-8 file contained the JSON escape \\u00e9. The test did not fail because of discovery, an import, or an old repository artifact.

Open the production file only after fixing those facts:

nano catalog.py

In write_catalog_json, change only the JSON option from ensure_ascii=True to ensure_ascii=False. That restores the inherited Chapter 7 through 9 contract: non-ASCII text stays visible in the UTF-8 artifact.

Rerun every test:

python -m pytest -q

Now collection is nonzero, every test passes, and pytest exits 0. Do not weaken the expectation to accept both escaped and visible text. A regression test is a safety net only while it continues to state the intended behavior.

Add one more tmp_path check for the existing failure boundary. Add run_catalog to the import from catalog, then use this small shape:

def test_missing_input_preserves_existing_output(tmp_path, capsys):
    missing = tmp_path / "missing.csv"
    output = tmp_path / "existing.json"
    output.write_text("keep me\n", encoding="utf-8")

    result = run_catalog(missing, output, 0)

    assert result == 1
    assert output.read_text(encoding="utf-8") == "keep me\n"
    assert capsys.readouterr().err.startswith(f"Could not read {missing}:")

Like tmp_path, capsys is supplied by pytest when you name it as a test parameter. capsys.readouterr() returns the captured output so far: .out holds stdout and .err holds stderr. Calling it also clears that captured buffer. Here the test inspects .err to check the expected diagnostic. Both file paths stay under tmp_path. The sentinel assertion checks the contract that matters: a read failure happens before the writer touches existing output.

Now prove the net still catches. Put ensure_ascii=True back, run the suite, and watch the same test go red again. Then set it to False and watch it go green.

That red-green-red-green cycle is the entire value of the test. A test you have only ever seen pass has not demonstrated anything yet; you have not watched it notice a problem.