code: def validate(self, value):
          
          if value is None:
              return
          if len(self.classes) and not isinstance(value, self.classes):
              msg = '"%s" must be: %s' % (self.name, self.classes)
              raise AttributeError(msg)
docs: Validate the I{value} is of the correct class.
          @param value: The value to validate.
          @type value: any
          @raise AttributeError: When I{value} is invalid.