diff --git a/zipline/gens/returns.py b/zipline/gens/returns.py
index a4932485..a407a1e0 100644
--- a/zipline/gens/returns.py
+++ b/zipline/gens/returns.py
@@ -1,6 +1,7 @@
 from zipline.gens.transform import TransformMeta
 from collections import defaultdict, deque
 
+
 class Returns(object):
     """
     Class that maintains a dictionary from sids to the sid's
@@ -16,8 +17,8 @@ class Returns(object):
         """
         Update and return the calculated returns for this event's sid.
         """
-        assert event.has_key('dt')
-        assert event.has_key('price')
+        assert 'dt' in event
+        assert 'price' in event
         tracker = self.mapping[event.sid]
         tracker.update(event)
 
@@ -26,6 +27,7 @@ class Returns(object):
     def _create(self):
         return ReturnsFromPriorClose(self.days)
 
+
 class ReturnsFromPriorClose(object):
     """
     Records the last N closing events for a given security as well as the
@@ -72,6 +74,5 @@ class ReturnsFromPriorClose(object):
             change = event.price - last_close
             self.returns = change / last_close
 
-
         # the current event is now the last_event
         self.last_event = event