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Sarj Sarj Sarj Standards

prefer-monkeypatch-for-process-state-in-test

Test mutates process-wide state without a restoring test scope.

SARJ446 Rule source Tests Catalog JSON

Why

The working directory and Python import registries are shared by every test in the process. A direct mutation can survive an assertion or setup failure, making later tests depend on execution order.

Fix

Establish restoration in setup with pytest's monkeypatch.chdir, monkeypatch.syspath_prepend, monkeypatch.setitem, or monkeypatch.delitem so the original state is recorded before the mutation. Alternatively, explicitly restore the original state with a matching try/finally. Adding monkeypatch operations only during teardown can undo intended cleanup: undoing a teardown deletion can reinstall a temporary sys.modules entry. Intentional cleanup with established ownership may use a local, reasoned SARJ446 suppression.

Examples

Before — flagged Test installs a module without guaranteed restoration
tests/test_plugin.py
import sys




def test_plugin(fake_plugin):
    sys.modules["optional_plugin"] = fake_plugin
After — preferred Pytest restores the module registry after the test
tests/test_plugin.py
import sys




def test_plugin(monkeypatch, fake_plugin):
    monkeypatch.setitem(sys.modules, "optional_plugin", fake_plugin)