Cache polymorphic_iterator in UOWTask; substantial savings for polymorphism-heavy workloads.

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/sqlalchemy/util.py b/lib/sqlalchemy/util.py
index 88245945e..770b48796 100644
--- a/lib/sqlalchemy/util.py
+++ b/lib/sqlalchemy/util.py
@@ -395,6 +395,19 @@ def iterate_attributes(cls):
                 yield (key, c.__dict__[key])
                 break
 
+class lazy_property(object):
+    """A read-only @property that is only evaluated once."""
+    def __init__(self, fget, doc=None):
+        self.fget = fget
+        self.__doc__ = doc or fget.__doc__
+        self.__name__ = fget.__name__
+
+    def __get__(self, obj, cls):
+        if obj is None:
+            return None
+        obj.__dict__[self.__name__] = result = self.fget(obj)
+        return result
+
 # from paste.deploy.converters
 def asbool(obj):
     if isinstance(obj, (str, unicode)):