Don’t forget self in ParkingLot levels

__levels__ is an _undefined name in this context (see #93) so replace it with __self.levels__ which is defined a few lines above.

solutions/object_oriented_design/parking_lot/parking_lot.py

@@ -64,7 +64,7 @@ class ParkingLot(object):
         self.levels = []  # List of Levels
 
     def park_vehicle(self, vehicle):
-        for level in levels:
+        for level in self.levels:
             if level.park_vehicle(vehicle):
                 return True
         return False