gistfile1.py

class Parser:
    def parse(self, text):
        self.onElement(text[::-1])

    def onElement(self, t):
        raise NotImplementedError


def cb_helper():
    value = yield
    yield None
    yield value

def test():
    p = Parser()

    helper = cb_helper()
    helper.next()
    p.onElement = helper.send

    p.parse('abc')

    value = helper.next()
    print 'Returned', value

test()