code: def register(self, observers):
          
  
          if isinstance(observers, list) or isinstance(observers, tuple):
  
              for observer in observers:
                  # check whether inhelitance "base.Observer"
                  if isinstance(observer, base.Observer):
                      self._observers.append(observer)
  
                  else:
                      raise InhelitanceError(base.Observer.__name__)
  
          elif isinstance(observers, base.Observer):
              self._observers.append(observers)
docs: Concrete method of Subject.register().
          Register observers as an argument to self.observers.