From b0d20b5c3fe7df472633899024cdab685483807a Mon Sep 17 00:00:00 2001
From: Amos Latteier <amos@latteier.com>
Date: Thu, 2 Jun 2016 14:50:53 -0700
Subject: add exception_only argument to add_view to only register exception
 views.

---
 pyramid/config/views.py                 | 25 +++++++++++++++++++++----
 pyramid/tests/test_config/test_views.py | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/pyramid/config/views.py b/pyramid/config/views.py
index 9e46ba155..198fde5e8 100644
--- a/pyramid/config/views.py
+++ b/pyramid/config/views.py
@@ -213,6 +213,7 @@ class ViewsConfiguratorMixin(object):
         match_param=None,
         check_csrf=None,
         require_csrf=None,
+        exception_only=False,
         **view_options):
         """ Add a :term:`view configuration` to the current
         configuration state.  Arguments to ``add_view`` are broken
@@ -701,6 +702,14 @@ class ViewsConfiguratorMixin(object):
              Support setting view deriver options. Previously, only custom
              view predicate values could be supplied.
 
+        exception_only
+
+          .. versionadded:: 1.8
+
+          A boolean indicating whether the view is registered only as an
+          exception view. When this argument is true, the view context must
+          be an exception.
+
         """
         if custom_predicates:
             warnings.warn(
@@ -759,6 +768,11 @@ class ViewsConfiguratorMixin(object):
                 raise ConfigurationError(
                     'request_type must be an interface, not %s' % request_type)
 
+        if exception_only and not isexception(context):
+            raise ConfigurationError(
+                'context must be an exception when exception_only is true'
+                )
+
         if context is None:
             context = for_
 
@@ -942,10 +956,13 @@ class ViewsConfiguratorMixin(object):
                     view_iface = ISecuredView
                 else:
                     view_iface = IView
-                self.registry.registerAdapter(
-                    derived_view,
-                    (IViewClassifier, request_iface, context), view_iface, name
-                    )
+                if not exception_only:
+                    self.registry.registerAdapter(
+                        derived_view,
+                        (IViewClassifier, request_iface, context),
+                        view_iface,
+                        name
+                        )
                 if isexc:
                     self.registry.registerAdapter(
                         derived_view,
diff --git a/pyramid/tests/test_config/test_views.py b/pyramid/tests/test_config/test_views.py
index 878574e88..c93175ff9 100644
--- a/pyramid/tests/test_config/test_views.py
+++ b/pyramid/tests/test_config/test_views.py
@@ -1815,6 +1815,38 @@ class TestViewsConfigurationMixin(unittest.TestCase):
 
         self.assertRaises(ConfigurationError, configure_view)
 
+    def test_add_view_exception_only_no_regular_view(self):
+        from zope.interface import implementedBy
+        from pyramid.renderers import null_renderer
+        view1 = lambda *arg: 'OK'
+        config = self._makeOne(autocommit=True)
+        config.add_view(view=view1, context=Exception, renderer=null_renderer,
+                        exception_only=True)
+        view = self._getViewCallable(config, ctx_iface=implementedBy(Exception))
+        self.assertTrue(view is None)
+
+    def test_add_view_exception_only(self):
+        from zope.interface import implementedBy
+        from pyramid.renderers import null_renderer
+        view1 = lambda *arg: 'OK'
+        config = self._makeOne(autocommit=True)
+        config.add_view(view=view1, context=Exception, renderer=null_renderer,
+                        exception_only=True)
+        view = self._getViewCallable(
+            config, ctx_iface=implementedBy(Exception), exception_view=True
+            )
+        self.assertEqual(view1, view)
+
+    def test_add_view_exception_only_misconfiguration(self):
+        view = lambda *arg: 'OK'
+        config = self._makeOne(autocommit=True)
+        class NotAnException(object):
+            pass
+        self.assertRaises(
+            ConfigurationError,
+            config.add_view, view, context=NotAnException, exception_only=True
+            )
+
     def test_derive_view_function(self):
         from pyramid.renderers import null_renderer
         def view(request):
-- 
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