Allow multiple target states by calling back

This should, however be used in very rare cases and transitions to single target
states be preferred.

concert/fsm.py

@@ -5,7 +5,11 @@
 
 
 class TransitionNotAllowed(Exception):
-    pass
+    def __init__(self, source, target):
+        self.value = "Invalid transition from {} to {}".format(source, target)
+
+    def __str__(self):
+        return repr(self.value)
 
 
 class Error(Exception):
@@ -96,6 +100,7 @@ def __get__(self, instance, owner):
 
 
 class Meta(object):
+
     def __init__(self):
         self.transitions = defaultdict()
         self.state_name = None
@@ -127,9 +132,18 @@ def do_transition(self, instance, target):
         except KeyError:
             next_state = self.transitions.get('*')
 
-        if current is None or next_state != target:
-            msg = "Invalid transition from {} to {}".format(current, target)
-            raise TransitionNotAllowed(msg)
+        if isinstance(next_state, list):
+            if not hasattr(instance, 'get_real_state'):
+                raise TypeError("{} must implement `get_real_state'".format(instance))
+
+            # Override target with actual, current device state
+            target = instance.get_real_state()
+
+            if target not in next_state:
+                raise TransitionNotAllowed(device_state, next_state)
+        else:
+            if current is None or next_state != target:
+                raise TransitionNotAllowed(current, target)
 
         attr = self.get_state_attribute(instance)
         attr._set_value(target)

concert/tests/unit/test_fsm.py

@@ -18,6 +18,9 @@ def __init__(self):
         super(SomeDevice, self).__init__()
         self.velocity = STOP_VELOCITY
 
+    def get_real_state(self):
+        return 'standby' if self.velocity == STOP_VELOCITY else 'moving'
+
     @transition(source='standby', target='moving')
     def start_moving(self, velocity):
         self.velocity = velocity
@@ -37,6 +40,10 @@ def move_some_time(self, velocity, duration):
     def stop_no_matter_what(self):
         self.velocity = STOP_VELOCITY
 
+    @transition(source='*', target=['standby', 'moving'])
+    def set_velocity(self, velocity):
+        self.velocity = velocity
+
     @transition(source='*')
     def cause_erroneous_behaviour(self, msg):
         raise Error(msg, self._fix_problem)
@@ -84,6 +91,13 @@ def test_multiple_source_states(self):
 
         self.assertTrue(self.device.state.is_currently('standby'))
 
+    def test_multiple_target_states(self):
+        self.device.set_velocity(MOVE_VELOCITY)
+        self.assertTrue(self.device.state.is_currently('moving'))
+
+        self.device.set_velocity(STOP_VELOCITY)
+        self.assertTrue(self.device.state.is_currently('standby'))
+
     def test_error(self):
         with self.assertRaises(RuntimeError):
             self.device.cause_erroneous_behaviour("Oops")