- Adjustment to attribute mechanics concerning when a value is

implicitly initialized to None via first access; this action,
which has always resulted in a population of the attribute,
now emits an attribute event just like any other attribute set
operation and generates the same kind of history as one.  Additionally,
many mapper internal operations will no longer implicitly generate
these "None" values when various never-set attributes are checked.
These are subtle behavioral fixes to attribute mechanics which provide
a better solution to the problem of :ticket:`3060`, which also
involves recognition of attributes explicitly set to ``None``
vs. attributes that were never set.
fixes #3061

1 files changed, 18 insertions, 14 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 09d6e988d..306c86e3b 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -23,7 +23,8 @@ from .base import PASSIVE_NO_RESULT, ATTR_WAS_SET, ATTR_EMPTY, NO_VALUE,\
             NEVER_SET, NO_CHANGE, CALLABLES_OK, SQL_OK, RELATED_OBJECT_OK,\
             INIT_OK, NON_PERSISTENT_OK, LOAD_AGAINST_COMMITTED, PASSIVE_OFF,\
             PASSIVE_RETURN_NEVER_SET, PASSIVE_NO_INITIALIZE, PASSIVE_NO_FETCH,\
-            PASSIVE_NO_FETCH_RELATED, PASSIVE_ONLY_PERSISTENT, NO_AUTOFLUSH
+            PASSIVE_NO_FETCH_RELATED, PASSIVE_ONLY_PERSISTENT, NO_AUTOFLUSH,\
+            _none_tuple
 from .base import state_str, instance_str
 
 @inspection._self_inspects
@@ -355,7 +356,10 @@ class Event(object):
         self.op = op
         self.parent_token = self.impl.parent_token
 
-
+    def __eq__(self, other):
+        return isinstance(other, Event) and \
+            other.impl is self.impl and \
+            other.op == self.op
     @property
     def key(self):
         return self.impl.key
@@ -553,8 +557,15 @@ class AttributeImpl(object):
     def initialize(self, state, dict_):
         """Initialize the given state's attribute with an empty value."""
 
-        dict_[self.key] = None
-        return None
+        old = NEVER_SET
+        value = None
+        if self.dispatch.set:
+            value = self.fire_replace_event(state, dict_,
+                                                None, old, None)
+        state._modified_event(dict_, self, old)
+
+        dict_[self.key] = value
+        return value
 
     def get(self, state, dict_, passive=PASSIVE_OFF):
         """Retrieve a value from the given object.
@@ -763,14 +774,7 @@ class ScalarObjectAttributeImpl(ScalarAttributeImpl):
         if self.dispatch._active_history:
             old = self.get(state, dict_, passive=PASSIVE_ONLY_PERSISTENT | NO_AUTOFLUSH)
         else:
-            # would like to call with PASSIVE_NO_FETCH ^ INIT_OK.  However,
-            # we have a long-standing behavior that a "get()" on never set
-            # should implicitly set the value to None.  Leaving INIT_OK
-            # set here means we are consistent whether or not we did a get
-            # first.
-            # see test_use_object_set_None vs. test_use_object_get_first_set_None
-            # in test_attributes.py
-            old = self.get(state, dict_, passive=PASSIVE_NO_FETCH)
+            old = self.get(state, dict_, passive=PASSIVE_NO_FETCH ^ INIT_OK)
 
         if check_old is not None and \
              old is not PASSIVE_NO_RESULT and \
@@ -1087,7 +1091,7 @@ def backref_listeners(attribute, key, uselist):
     def emit_backref_from_scalar_set_event(state, child, oldchild, initiator):
         if oldchild is child:
             return child
-        if oldchild is not None and oldchild not in (PASSIVE_NO_RESULT, NEVER_SET):
+        if oldchild not in _none_tuple:
             # With lazy=None, there's no guarantee that the full collection is
             # present when updating via a backref.
             old_state, old_dict = instance_state(oldchild),\
@@ -1175,7 +1179,7 @@ _NO_STATE_SYMBOLS = frozenset([
 
 History = util.namedtuple("History", [
         "added", "unchanged", "deleted"
-    ])
+])
 
 
 class History(History):