Document a possible attribute cache issue with the proposed changes to PyObject_GenericGetAttr

pep-0447.txt

@@ -7,7 +7,7 @@ Status: Draft
 Type: Standards Track
 Content-Type: text/x-rst
 Created: 12-Jun-2013
-Post-History: 2-Jul-2013, ?
+Post-History: 2-Jul-2013, 15-Jul-2013
 
 
 Abstract
@@ -137,6 +137,17 @@ Open Issues
   slightly faster because it doesn't have to overhead of setting up exception state, but
   makes it impossible to use that value as an attribute on a class.
 
+* The proposed change to `PyObject_GenericGetAttr`_ will probably cause problems with the
+  attribute lookup cache (MCACHE):
+
+  1. That code stores borrowed references, which won't work when the hook is present. That
+     is mostly fixable, but at the cost of possibly keeping garbage alive.
+
+  2. Caching isn't an option when a hook might execute arbitrary code (and there hence is
+     no reason to assume that the hooks return value won't change later on).
+
+     The only workaround I could find for this is to make the hook a fallback (that is,
+     more like ``__getattr__`` than ``__getattribute__``).
 
 References
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