Test issue #1542

openff/toolkit/tests/test_forcefield.py

@@ -1960,6 +1960,19 @@ class BogusHandler(ParameterHandler):
         assert force_field["bogus"] is not None
 
 
+class TestForceFieldPluginLoading:
+    def test_handlers_tracked_if_already_loaded(self):
+        """Reproduce issue #1542."""
+        from openff.toolkit.typing.engines.smirnoff.plugins import load_handler_plugins
+
+        plugins = load_handler_plugins()
+
+        # This might need to be updated if we ship, by default, other plugins
+        assert len(plugins) == 1, "that 1 ParameterHandler plugin is available"
+
+        assert ForceField()._plugin_parameter_handler_classes == plugins
+
+
 class TestForceFieldSerializaiton(_ForceFieldFixtures):
     def test_json_dump(self, force_field):
         """Test that at a ForceField can at least be dumped to JSON without error."""