code: def attr(prev, attr_name):
      
      for obj in prev:
          if hasattr(obj, attr_name):
              yield getattr(obj, attr_name)
docs: attr pipe can extract attribute value of object.
  
      :param prev: The previous iterator of pipe.
      :type prev: Pipe
      :param attr_name: The name of attribute
      :type attr_name: str
      :returns: generator