tests: improve assertion when forbidden events occur

This makes the backtraces prettier; it doesn't have any functional
effect.

1 files changed, 9 insertions, 4 deletions

diff --git a/tests/twisted/servicetest.py b/tests/twisted/servicetest.py
index 700c05ad..36ff6683 100644
--- a/tests/twisted/servicetest.py
+++ b/tests/twisted/servicetest.py
@@ -93,6 +93,14 @@ class EventPattern:
 class TimeoutError(Exception):
     pass
 
+class ForbiddenEventOccurred(Exception):
+    def __init__(self, event):
+        Exception.__init__(self)
+        self.event = event
+
+    def __str__(self):
+        return '\n' + '\n'.join(format_event(self.event))
+
 class BaseEventQueue:
     """Abstract event queue base class.
 
@@ -155,10 +163,7 @@ class BaseEventQueue:
     def _check_forbidden(self, event):
         for e in self.forbidden_events:
             if e.match(event):
-                print "forbidden event occurred:"
-                for x in format_event(event):
-                    print x
-                assert False
+                raise ForbiddenEventOccurred(event)
 
     def expect(self, type, **kw):
         pattern = EventPattern(type, **kw)