Fix check for valid handler

src/hsm/core.py

@@ -369,7 +369,7 @@ def _on(self, event):
                 )
             )
 
-        if handler:
+        if handler is not None:
             if e is not None:
                 e.propagate = False
             _call(handler, e)

test/test_basic.py

@@ -0,0 +1,24 @@
+from __future__ import print_function
+
+import hsm
+
+
+class SM(hsm.StateMachine):
+    def __init__(self):
+        super(SM, self).__init__("SM")
+        self.A = self.add_state("A", initial=True)
+        self.B = self.add_state("B")
+
+        self.A.add_handler("event", self.handler)
+        self.A.add_transition
+        self.initialize()
+
+    def handler(self, event):
+        print("Processing event '{}' in state '{}'".format(event.name, self.leaf_state.name))
+        self._transition_to(self.B, event)
+
+
+if __name__ == "__main__":
+    sm = SM()
+    sm.dispatch("event")
+    sm.dispatch("event")