Iterate over input and call the mapper for each item.
        If the job has a parser defined, the return values from the parser will
        be passed as arguments to the mapper.

        If the input is coded output from a previous run,
        the arguments will be splitted in key and value.

def _map_input(self, input_stream):
        """
        Iterate over input and call the mapper for each item.
        If the job has a parser defined, the return values from the parser will
        be passed as arguments to the mapper.

        If the input is coded output from a previous run,
        the arguments will be splitted in key and value.
        """
        for record in self.reader(input_stream):
            for output in self.mapper(*record):
                yield output
        if self.final_mapper != NotImplemented:
            for output in self.final_mapper():
                yield output
        self._flush_batch_incr_counter()