'Get the reader from mapper_spec initialized with the request\'s state.
Args:
mapper_spec: a mapper spec containing the immutable mapper state.
Returns:
An initialized InputReader.'

def input_reader(self, mapper_spec):

    input_reader_spec_dict = simplejson.loads(self.request.get('input_reader_state'))
    return mapper_spec.input_reader_class().from_json(input_reader_spec_dict)
