2 files changed, 22 insertions, 1 deletions
diff --git a/gerrit_stream.py b/gerrit_stream.py
index 5578f7f..00c74ed 100755
--- a/gerrit_stream.py
+++ b/gerrit_stream.py
@@ -227,11 +227,13 @@ class GerritStream:
     def attach(self, listener):
         ''' Attach the `listener` to the list of listeners.
         Raise GerritStream error if the listener does not match the
-        expected signature.
+        expected signature, or if its event handler is not callable.
         '''
         if not hasattr(listener, "on_gerrit_event"):
             raise GerritStreamError("Listener must have `on_gerrit_event` " \
                                     "event handler")
+        if not callable(listener.on_gerrit_event):
+            raise GerritStreamError("`on_gerrit_event` must be callable")
         if not listener.on_gerrit_event.func_code.co_argcount == 2:
             raise GerritStreamError("`on_gerrit_event` must take 1 arg")
         if not listener in self.listeners:
diff --git a/tests/gerrit_stream/unittest_gerrit_stream/unittests.py b/tests/gerrit_stream/unittest_gerrit_stream/unittests.py
index c84c932..93f9925 100755
--- a/tests/gerrit_stream/unittest_gerrit_stream/unittests.py
+++ b/tests/gerrit_stream/unittest_gerrit_stream/unittests.py
@@ -23,6 +23,17 @@ class ListenerWithInvalidHandler():
         pass
 
 
+class ListenerWithInvalidHandlerNotCallable():
+    """ Dummy listener class with invalid event handler
+    that is not callable.
+    """
+
+    on_gerrit_event = "this is a string"
+
+    def __init__(self):
+        pass
+
+
 class ListenerWithValidHandler():
     """ Dummy listener class.
     """
@@ -53,6 +64,14 @@ class TestGerritStream(unittest.TestCase):
         l = ListenerWithInvalidHandler()
         self.assertRaises(GerritStreamError, g.attach, l)
 
+    def test_listener_non_callable_handler(self):
+        """ Test that an exception is raised if a listener with
+        non-callable event handler is added.
+        """
+        g = GerritStream()
+        l = ListenerWithInvalidHandlerNotCallable()
+        self.assertRaises(GerritStreamError, g.attach, l)
+
     def test_listener_valid_handler(self):
         """ Test that a valid listener can be added.
         """