code: def to_python(self, reply, propagate=True):
          
          try:
              return reply['ok']
          except KeyError:
              error = self.Error(*reply.get('nok') or ())
              if propagate:
                  raise error
              return error
docs: Extracts the value out of the reply message.
  
          :param reply: In the case of a successful call the reply message
              will be::
  
                  {'ok': return_value, **default_fields}
  
              Therefore the method returns: return_value, **default_fields
  
              If the method raises an exception the reply message
              will be::
  
                  {'nok': [repr exc, str traceback], **default_fields}
  
          :keyword propagate - Propagate exceptions raised instead of returning
              a result representation of the error.