code: def keys(self):
          "Returns all the keys this object can return proper values for."
          return tuple(set(self.new.keys()).union(self.old.keys())f keys(self):
          "Returns all the keys this object can return proper values for."
          return tuple(set(self.new.keys()).union(self.old.keys()))
docs: Returns all the keys this object can return proper values for.