test proper subclassing for hierarchicalstatemachine (#510)

Changelog.md

@@ -7,6 +7,7 @@
 - Bugfix  #512: Use `model_attribute` consistently in `AsyncMachine` (thanks @thedrow)
 - Testing now treats most warnings as errors (thanks @thedrow)
 - As a consequence, `pygraphviz.Agraph` in `diagrams_pygraphviz` are now copied by `transitions` since `AGraph.copy` as of version `1.6` does not close temporary files appropriately
+- `HierarchicalMachine` now checks whether `state_cls`, `event_cls` and `transition_cls` have been subclassed from nested base classes (e.g. `NestedState`) to prevent hard to debug inheritance errors
 
 ## 0.8.6 (December 2020)
 

tests/test_nesting.py

@@ -570,6 +570,22 @@ def on_exit_C_1(self):
         model.to_C()
         self.assertTrue(model.is_C_1())
 
+    def test_correct_subclassing(self):
+        from transitions.core import State
+
+        class WrongStateClass(self.machine_cls):
+            state_cls = State
+
+        class MyNestedState(NestedState):
+            pass
+
+        class CorrectStateClass(self.machine_cls):
+            state_cls = MyNestedState
+
+        with self.assertRaises(AssertionError):
+            m = WrongStateClass()
+        m = CorrectStateClass()
+
 
 class TestSeparatorsBase(TestCase):
 

tests/test_nesting_legacy.py

@@ -100,6 +100,9 @@ def test_child_condition_persistence(self):
     def test_get_nested_transitions(self):
         pass  # not supported by legacy machine
 
+    def test_correct_subclassing(self):
+        pass  # not supported by legacy machine
+
 
 class TestReuseLegacySeparatorDefault(TestReuseSeparatorBase):
 

transitions/extensions/nesting.py

@@ -334,6 +334,9 @@ class HierarchicalMachine(Machine):
     event_cls = NestedEvent
 
     def __init__(self, *args, **kwargs):
+        assert issubclass(self.state_cls, NestedState)
+        assert issubclass(self.event_cls, NestedEvent)
+        assert issubclass(self.transition_cls, NestedTransition)
         self._stack = []
         self.scoped = self
         _super(HierarchicalMachine, self).__init__(*args, **kwargs)