diff --git a/octavia_lib/api/drivers/data_models.py b/octavia_lib/api/drivers/data_models.py
index aefa56c..b017979 100644
--- a/octavia_lib/api/drivers/data_models.py
+++ b/octavia_lib/api/drivers/data_models.py
@@ -22,10 +22,9 @@ class BaseDataModel():
         """Converts a data model to a dictionary."""
         calling_classes = calling_classes or []
         ret = {}
-        for attr in self.__dict__:
+        for attr, value in self.__dict__.items():
             if attr.startswith('_') or not kwargs.get(attr, True):
                 continue
-            value = self.__dict__[attr]
 
             if recurse:
                 if isinstance(getattr(self, attr), list):
diff --git a/tox.ini b/tox.ini
index 7afad97..3d85025 100644
--- a/tox.ini
+++ b/tox.ini
@@ -43,7 +43,9 @@ commands =
     coverage report --fail-under=95 --skip-covered
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 whitelist_externals = rm
 commands =
   rm -rf doc/build doc/source/reference/modules