Add sanity check to receives decorator

Add simple assertion to prevent simple mistakes like
I4a39f2e9d2f10639a209e7311fe44b94d369c5b1.

Change-Id: I3a18da4949dcd97096817c5e157cac99bb37c04d

neutron_lib/callbacks/registry.py

@@ -68,6 +68,8 @@ def receives(resource, events):
     @has_registry_receivers decorator to setup the __new__ method to
     actually register the instance methods after initialization.
     """
+    assert isinstance(events, (list, tuple, set)), 'events must be collection'
+
     def decorator(f):
         for e in events:
             _REGISTERED_CLASS_METHODS[f].append((resource, e))

neutron_lib/tests/unit/callbacks/test_registry.py

@@ -13,6 +13,7 @@
 #    under the License.
 
 import mock
+import testtools
 
 from oslotest import base
 
@@ -99,6 +100,10 @@ class CallBacksManagerTestCase(base.BaseTestCase):
     def test_object_new_not_broken(self):
         CallbackClassWithParameters('dummy')
 
+    def test_no_strings_in_events_arg(self):
+        with testtools.ExpectedException(AssertionError):
+            registry.receives(resources.PORT, events.AFTER_CREATE)
+
 
 class TestCallbackRegistryDispatching(base.BaseTestCase):