Merge pull request #836 from davidjb/public-predicate-mismatch

Document PredicateMismatch for exception contexts

docs/api/exceptions.rst

@@ -5,6 +5,8 @@
 
 .. automodule:: pyramid.exceptions
 
+  .. autoclass:: PredicateMismatch
+
   .. autoclass:: Forbidden
 
   .. autoclass:: NotFound

pyramid/exceptions.py

@@ -10,11 +10,29 @@
 
 class PredicateMismatch(HTTPNotFound):
     """
-    Internal exception (not an API) raised by multiviews when no
-    view matches.  This exception subclasses the ``NotFound``
-    exception only one reason: if it reaches the main exception
-    handler, it should be treated like a ``NotFound`` by any exception
-    view registrations.
+    This exception is raised by multiviews when no view matches
+    all given predicates.
+
+    This exception subclasses the :class:`HTTPNotFound` exception for a
+    specific reason: if it reaches the main exception handler, it should
+    be treated as :class:`HTTPNotFound`` by any exception view
+    registrations. Thus, typically, this exception will not be seen
+    publicly.
+    
+    However, this exception will be raised if the predicates of all
+    views configured to handle another exception context cannot be
+    successfully matched.  For instance, if a view is configured to
+    handle a context of ``HTTPForbidden`` and the configured with
+    additional predicates, then :class:`PredicateMismatch` will be
+    raised if:
+
+    * An original view callable has raised :class:`HTTPForbidden` (thus
+      invoking an exception view); and
+    * The given request fails to match all predicates for said
+      exception view associated with :class:`HTTPForbidden`.
+
+    The same applies to any type of exception being handled by an
+    exception view.
     """
 
 class URLDecodeError(UnicodeDecodeError):