code: def CheckValue(self, proposed_value):
      
      if not isinstance(proposed_value, self._acceptable_types):
        message = ('%.1024r has type %s, but expected one of: %s' %
                   (proposed_value, type(proposed_value), self._acceptable_types))
        raise TypeError(message)
      return proposed_value
docs: Type check the provided value and return it.
  
      The returned value might have been normalized to another type.