The memo cache key needs to include the policy

Externalizing the same object twice using different policies probably doesn't happen, but might.

A bug in the test was hiding the fact that it did not work.

src/nti/externalization/externalization/externalizer.py

@@ -257,11 +257,12 @@ def _to_external_object_state(obj, state, top_level=False):
     assert obj is not None # caught by primitives already.
 
     orig_obj_id = id(obj) # XXX: Relatively expensive on PyPy
+    cache_key = (orig_obj_id, state.policy)
     if state.useCache:
-        value = state.memo.get(orig_obj_id, None)
+        value = state.memo.get(cache_key, None)
         result = value[1] if value is not None else None
         if result is None:  # mark as in progress
-            state.memo[orig_obj_id] = (obj, _marker)
+            state.memo[cache_key] = (obj, _marker)
         elif result is not _marker:
             return result
         else:
@@ -306,7 +307,7 @@ def _to_external_object_state(obj, state, top_level=False):
         )
 
         if state.useCache:  # save result
-            state.memo[orig_obj_id] = (obj, result)
+            state.memo[cache_key] = (obj, result)
         return result
     except state.catch_components as t:
         if top_level or state.catch_component_action is None:
@@ -399,6 +400,7 @@ def to_external_object(
             FutureWarning
         )
 
+
     if policy is NotGiven:
         if policy_name is not NotGiven:
             policy = getUtility(IExternalizationPolicy, policy_name)

src/nti/externalization/tests/test_externalization.py

@@ -423,12 +423,14 @@ def child(self, new):
 
         def toExternalObject(self, *args, **kwargs):
             result = TestExternalizableInstanceDict.X.toExternalObject(self, *args, **kwargs)
-            # We deliberately don't pass the kwargs to the
-            # parent, in order to test the thread-local storage.
-            # (Or the policy_name)
             if self.policy_name:
                 result['child'] = toExternalObject(self._x, policy_name=self.policy_name)
-            result['child'] = toExternalObject(self._x)
+            else:
+                # We deliberately don't pass the kwargs to the
+                # function, in order to test the thread-local storage.
+                # (Or the policy_name)
+                result['child'] = toExternalObject(self._x)
+            result['default_child'] = toExternalObject(self._x)
             return result
 
     def test_simple_roundtrip(self):
@@ -500,9 +502,15 @@ def test_registered_policy_name(self):
             assert_that(ext['Last Modified'], is_(x.lastModified))
 
             # child used ISO policy
-            ext = ext['child']
-            assert_that(ext['CreatedTime'], is_(self.created_string))
-            assert_that(ext['Last Modified'], is_(self.modified_string))
+            child = ext['child']
+            assert_that(child['CreatedTime'], is_(self.created_string))
+            assert_that(child['Last Modified'], is_(self.modified_string))
+
+            # next externalization used the default again
+            child = ext['default_child']
+            assert_that(child['CreatedTime'], is_(x.createdTime))
+            assert_that(child['Last Modified'], is_(x.lastModified))
+
         finally:
             component.getSiteManager().unregisterUtility(iso_policy, name='iso_policy')