PY-TEST0008 · direct_shared_test_state_mutation_count

Count direct mutations of module-owned state by collected pytest tests.

This is a deterministic rule for python. Read its implementation.

Definition

Inspect functions and methods collected by pytest’s default Python conventions. Report direct assignment, deletion, augmented assignment, or known mutable-container operations whose root name belongs to the test module rather than the test’s local scope. An explicit global assignment is shared state. A fixture parameter or local binding with the same name is local and is not reported.

Evidence

Each finding identifies the collected test, mutation site, and shared root. The rule proves a direct syntax-level mutation. It does not guess whether an arbitrary called helper has hidden side effects. The value is the number of direct mutations of module-owned state.

Exceptions

Fresh values supplied by function-scoped fixtures are accepted, as are mutations of ordinary local variables. Use pytest’s monkeypatch fixture for process state and yield fixtures for state that needs explicit teardown. Ruff’s PT family checks fixture syntax, not cross-test state ownership.

Examples

Bad

REQUESTS = []

def test_request_is_recorded():
    REQUESTS.append("/health")
    assert REQUESTS == ["/health"]

Good

@pytest.fixture
def requests():
    return []

def test_request_is_recorded(requests):
    requests.append("/health")
    assert requests == ["/health"]

References

• Cites “pytest documentation”, fixture reuse and fresh per-test values. Open reference
• Cites “pytest documentation”, monkeypatch guidance. Open reference
• Cites “pytest documentation”, default collection conventions. Open reference