@pipe.func<EOL>def attr(prev, attr_name):
for obj in prev:<EOL><INDENT>if hasattr(obj, attr_name):<EOL><INDENT>yield getattr(obj, attr_name)<EOL><DEDENT><DEDENT>
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