override_containers: delete afterwards when type was not there.

And add tests for this context manager.

src/AccessControl/SimpleObjectPolicies.py

@@ -44,6 +44,7 @@
 '''
 
 _noroles = [] # this is imported in various places
+_marker = object()
 
 from contextlib import contextmanager
 import Record
@@ -125,8 +126,10 @@ def allow_type(Type, allowed=1):
 @contextmanager
 def override_containers(type_, assertions):
     """Temporarily override the container assertions."""
-    orig_container = Containers(type_)
+    orig_container = Containers(type_, _marker)
     ContainerAssertions[type_] = assertions
     yield
-    if orig_container is not None:
+    if orig_container is _marker:
+        del ContainerAssertions[type_]
+    else:
         ContainerAssertions[type_] = orig_container

src/AccessControl/tests/testZopeSecurityPolicy.py

@@ -335,6 +335,27 @@ def testUnicodeRolesForPermission(self):
         v = self.policy.checkPermission(u'View', r_item, o_context)
         self.assert_(v, '_View_Permission should grant access to theowner')
 
+    def testContainersContextManager(self):
+        from AccessControl.SimpleObjectPolicies import override_containers
+        from AccessControl.SimpleObjectPolicies import ContainerAssertions
+        from AccessControl.SimpleObjectPolicies import Containers
+        from types import EllipsisType
+        # Surely we have no assertions for this type.  There might be a good
+        # reason to have then, but I have not even heard of this type.
+        self.assertFalse(EllipsisType in ContainerAssertions)
+        with override_containers(EllipsisType, 1):
+            self.assertTrue(EllipsisType in ContainerAssertions)
+            self.assertEqual(Containers(EllipsisType), 1)
+            # Override it again.
+            with override_containers(EllipsisType, {}):
+                self.assertEqual(Containers(EllipsisType), {})
+            # We are outside the nested override, so the first override should
+            # have been restored.
+            self.assertEqual(Containers(EllipsisType), 1)
+        # We are outside all overrides, so the type should no longer be in the
+        # assertions.
+        self.assertFalse(EllipsisType in ContainerAssertions)
+
     def testAqNames(self):
         policy = self.policy
         names = {