Hold implicitly created collections in a pending area

Accessing a collection-oriented attribute on a newly created object no
longer mutates ``__dict__``, but still returns an empty collection as has
always been the case.   This allows collection-oriented attributes to work
consistently in comparison to scalar attributes which return ``None``, but
also don't mutate ``__dict__``.  In order to accommodate for the collection
being mutated, the same empty collection is returned each time once
initially created, and when it is mutated (e.g. an item appended, added,
etc.) it is then moved into ``__dict__``.  This removes the last of
mutating side-effects on read-only attribute access within the ORM.

Fixes: #4519
Change-Id: I06a058d24e6eb24b5c6b6092d3f8b31cf9c244ae

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/state.py b/lib/sqlalchemy/orm/state.py
index c6252b6b8..f6c06acc8 100644
--- a/lib/sqlalchemy/orm/state.py
+++ b/lib/sqlalchemy/orm/state.py
@@ -311,6 +311,10 @@ class InstanceState(interfaces.InspectionAttrInfo):
         return {}
 
     @util.memoized_property
+    def _empty_collections(self):
+        return {}
+
+    @util.memoized_property
     def mapper(self):
         """Return the :class:`.Mapper` used for this mapped object."""
         return self.manager.mapper