add test; fix c optimizations

src/zope/interface/_zope_interface_coptimizations.c

@@ -906,12 +906,16 @@ _lookup(lookup *self,
           return NULL;
         }
 
-      if (result == Py_None && default_ != NULL)
+      if (result == Py_None)
         {
           /* do not cache misses because the name is not included in the cache key */
+          if (default_ != NULL)
+            {
+              Py_INCREF(default_);
+              return default_;
+            }
           Py_DECREF(Py_None);
-          Py_INCREF(default_);
-          return default_;
+          return result;
         }
       status = PyDict_SetItem(cache, key, result);
       Py_DECREF(required);

src/zope/interface/tests/test_adapter.py

@@ -94,17 +94,17 @@ def test_multi_adapter_lookupAll_get_best_matches():
 def test_multi_adapter_w_default():
     """
     >>> registry = AdapterRegistry()
-    
+
     >>> registry.register([None, None], IB1, 'bob', 'A0')
 
     >>> registry.lookup((IF1, IR1), IB0, 'bob')
     'A0'
-    
+
     >>> registry.register([None, IR0], IB1, 'bob', 'A1')
 
     >>> registry.lookup((IF1, IR1), IB0, 'bob')
     'A1'
-    
+
     >>> registry.lookup((IF1, IR1), IB0, 'bruce')
 
     >>> registry.register([None, IR1], IB1, 'bob', 'A2')
@@ -192,7 +192,7 @@ def test_adapter_hook_with_factory_producing_None():
     """
     >>> registry = AdapterRegistry()
     >>> default = object()
-    
+
     >>> class Object1(object):
     ...     zope.interface.implements(IF0)
     >>> class Object2(object):
@@ -293,6 +293,18 @@ def test_changing_declarations():
     42
     """
 
+def test_lookup_failure():
+    """
+    >>> registry = AdapterRegistry()
+    >>> registry.register((), IF0, '', 42)
+    >>> def subsequent_lookup():
+    ...     print registry.lookup((), IF0, None)
+    ...     print registry.lookup((), IF0)
+    >>> subsequent_lookup()
+    None
+    42
+    """
+
 def test_correct_multi_adapter_lookup():
     """
     >>> registry = AdapterRegistry()