Consider transitions.dest is None for Machine._can_trigger

This fixes #594

Author: aleneum

Changelog.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## 0.9.1 ()
+
+- Bug #594: Fix may_<trigger> always returning false for internal transitions (thanks @a-schade)
+
 ## 0.9.0 (September 2022)
 
 Release 0.9.0 is a major release and contains improvements to ease development, adds some new features and removes the legacy hierarchical machine:

tests/test_core.py

@@ -1229,12 +1229,15 @@ def test_may_transition(self):
         m = Machine(model=d, states=states, initial='A', auto_transitions=False)
         m.add_transition('walk', 'A', 'B')
         m.add_transition('stop', 'B', 'C')
+        m.add_transition('wait', 'B', None)
         assert d.may_walk()
         assert not d.may_stop()
+        assert not d.may_wait()
 
         d.walk()
         assert not d.may_walk()
         assert d.may_stop()
+        assert d.may_wait()
 
     def test_may_transition_for_autogenerated_triggers(self):
         states = ['A', 'B', 'C']

transitions/core.py

@@ -884,7 +884,7 @@ def _can_trigger(self, model, trigger, *args, **kwargs):
                 continue
             for transition in self.events[trigger_name].transitions[state]:
                 try:
-                    _ = self.get_state(transition.dest)
+                    _ = transition.source if transition.dest is None else self.get_state(transition.dest)
                 except ValueError:
                     continue