pytester

# Pytester Guide for pytester-based plugin/CLI tests. Run before writing or modifying these tests. ## Which execution mode to use - **`runpytest()`** — default. In-process, fast, full `RunResult` API (`assert_outcomes()`, `fnmatch_lines()`). - **`runpytest_subprocess()`** — use only when in-process causes state leakage (Pydantic cache pollution, global mutation in `pytest_configure`). Same `RunResult` API. - **Raw `subprocess.run()`** — never use alongside pytester. Use `runpytest_subprocess()` instead. Subprocess isolation masks bugs rather than fixing them. Prefer fixing the root cause and use subprocess as defense-in-depth. ## Expected inner failures `runpytest_subprocess()` replays inner output to outer stdout (by design). Suppress with `capsys.readouterr()`: ```python def test_expected_failure(pytester: Any, capsys: Any, pytestconfig: Any) -> None: result = pytester.runpytest_subprocess(...) capsys.readouterr() # suppress inner failure bleed assert result.ret != 0 output = "\n".join(result.outlines + result.errlines) # conditional print for -s debugging if pytestconfig.getoption("capture") == "no": with capsys.disabled(): print(output) ``` ## RunResult API Prefer `assert_outcomes()` and `fnmatch_lines()` over manual `any(... in line ...)` — better failure messages. - `result.ret` — exit code - `result.outlines` / `result.errlines` — line lists - `result.assert_outcomes(passed=N, failed=N)` - `result.stdout.fnmatch_lines(["*pattern*"])`