'Initialize an Attribute object, used to specify the
attribute to consider, for a given node.'

def __init__(self, key, multi=False, path=None, joiner=None, postprocess=None):

    self.key = key
    self.multi = multi
    self.path = path
    if (joiner is None):
        joiner = ''
    self.joiner = joiner
    self.postprocess = postprocess
