From 5e3de90c7d948bb82e4651e9d5b73a63472fa9e9 Mon Sep 17 00:00:00 2001
From: Aurélien Bompard <aurelien@bompard.org>
Date: Feb 10 2017 10:27:24 +0000
Subject: Make CachedFunction.should_invalidate() abstract


---

diff --git a/hubs/widgets/caching.py b/hubs/widgets/caching.py
index 9067410..990dce6 100644
--- a/hubs/widgets/caching.py
+++ b/hubs/widgets/caching.py
@@ -88,7 +88,8 @@ class CachedFunction(object):
     def should_invalidate(self, message):
         """
         Tell the cache invalidator if the received message should invalidate
-        this function's cache.
+        this function's cache.  This function must be implemented by
+        subclasses.
 
         Args:
             message (dict): The recieved bus message.
@@ -96,7 +97,7 @@ class CachedFunction(object):
         Returns:
             bool: If ``True``, the cache will be rebuilt.
         """
-        return True
+        raise NotImplementedError
 
     def is_cached(self):
         """