`--collect-only` executes tests when pytest-isolated is installed (missing collectonly guard in custom runtestloop)

[pcrespov]

Summary

With pytest-isolated installed, pytest --collect-only runs tests instead of only collecting them. This is not limited to @pytest.mark.isolated tests: the mere presence of the active plugin breaks collect-only for the entire session, including unmarked tests. Disabling the plugin (-p no:isolated) restores correct behavior.

Environment

• pytest-isolated: 0.4.3
• pytest: 9.0.2
• Python: 3.11 / 3.12 (reproduced on 3.12.3; root cause is version-independent)
• OS: Linux

Reproducer

test_repro.py:

import pathlib, pytest

def test_plain():            # NOT marked isolated
    pathlib.Path("plain.ran").write_text("x")
    assert True

@pytest.mark.isolated
def test_isolated():
    pathlib.Path("isolated.ran").write_text("x")
    assert True

Behavior matrix (file markers prove execution)

command	plain.ran	isolated.ran
pytest --collect-only	YES	YES
pytest --collect-only --no-isolation	YES	YES
pytest --co -q	YES	YES
pytest -p no:isolated --collect-only	no	no

A file containing zero isolated markers also runs its tests under --collect-only as long as the plugin is active, confirming the trigger is the plugin, not the marker.

Expected

--collect-only lists items and executes nothing (matching stock pytest, i.e. -p no:isolated).

Root cause (assumption ... please verify)

pytest_isolated/execution.py::pytest_runtestloop replaces pytest's built-in loop but omits the collect-only short-circuit. Stock _pytest.main.pytest_runtestloop early-returns when session.config.option.collectonly is set; the replacement only guards against the subprocess env var:

def pytest_runtestloop(session):
    if os.environ.get(SUBPROC_ENV) == "1":
        return None
    # <-- no `if session.config.option.collectonly: return True` guard
    ...  # proceeds to execute groups + remaining items

Suggested fix

Add the standard guard at the top of the custom loop:

if session.config.option.collectonly:
    return True

(Mirrors _pytest.main.pytest_runtestloop. Returning True marks the hook as
handled and skips execution.)

Workaround

pytest -p no:isolated --collect-only until fixed.