Handle case where InstanceState.obj returns None

Fixed bug where a call ``InstanceState.obj()`` could return None when
synchronizing the instance states of the objects in the session in case
they become out of scope but are not yet finalized by the gc. This
case does not happen in cPython, but it may present itself in pypy.

The approach is to allow None to be gracefully handled by the
evaluator itself, ensuring it returns None in all cases when None
is passed in.

Fixes: #5631
Change-Id: I53d38fbea2e72b2e677c6e7f70bf075a58e58945

1 files changed, 21 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/evaluator.py b/lib/sqlalchemy/orm/evaluator.py
index f7f12ce12..23c48329d 100644
--- a/lib/sqlalchemy/orm/evaluator.py
+++ b/lib/sqlalchemy/orm/evaluator.py
@@ -17,6 +17,16 @@ class UnevaluatableError(Exception):
     pass
 
 
+class _NoObject(operators.ColumnOperators):
+    def operate(self, *arg, **kw):
+        return None
+
+    def reverse_operate(self, *arg, **kw):
+        return None
+
+
+_NO_OBJECT = _NoObject()
+
 _straight_ops = set(
     getattr(operators, op)
     for op in (
@@ -36,8 +46,10 @@ _straight_ops = set(
 )
 
 _extended_ops = {
-    operators.in_op: (lambda a, b: a in b),
-    operators.not_in_op: (lambda a, b: a not in b),
+    operators.in_op: (lambda a, b: a in b if a is not _NO_OBJECT else None),
+    operators.not_in_op: (
+        lambda a, b: a not in b if a is not _NO_OBJECT else None
+    ),
 }
 
 _notimplemented_ops = set(
@@ -111,7 +123,11 @@ class EvaluatorCompiler(object):
                 raise UnevaluatableError("Cannot evaluate column: %s" % clause)
 
         get_corresponding_attr = operator.attrgetter(key)
-        return lambda obj: get_corresponding_attr(obj)
+        return (
+            lambda obj: get_corresponding_attr(obj)
+            if obj is not None
+            else _NO_OBJECT
+        )
 
     def visit_tuple(self, clause):
         return self.visit_clauselist(clause)
@@ -137,7 +153,7 @@ class EvaluatorCompiler(object):
                 for sub_evaluate in evaluators:
                     value = sub_evaluate(obj)
                     if not value:
-                        if value is None:
+                        if value is None or value is _NO_OBJECT:
                             return None
                         return False
                 return True
@@ -148,7 +164,7 @@ class EvaluatorCompiler(object):
                 values = []
                 for sub_evaluate in evaluators:
                     value = sub_evaluate(obj)
-                    if value is None:
+                    if value is None or value is _NO_OBJECT:
                         return None
                     values.append(value)
                 return tuple(values)