Add an init_scalar event for attributes

This allows us to build default-setting recipes such
as one that allows us to actively read column-level
defaults.  An example suite is also added.

Change-Id: I7b022d52cc89526132d5bc4201ac27fea4cf088d
Fixes: #1311

1 files changed, 6 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 017ad0300..7239d41f2 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -551,12 +551,13 @@ class AttributeImpl(object):
     def initialize(self, state, dict_):
         """Initialize the given state's attribute with an empty value."""
 
-        # As of 1.0, we don't actually set a value in
-        # dict_.  This is so that the state of the object does not get
-        # modified without emitting the appropriate events.
+        value = None
+        for fn in self.dispatch.init_scalar:
+            ret = fn(state, value, dict_)
+            if ret is not ATTR_EMPTY:
+                value = ret
 
-
-        return None
+        return value
 
     def get(self, state, dict_, passive=PASSIVE_OFF):
         """Retrieve a value from the given object.