Unpack parts from payload

def unpack_from(cls, payload, expected_parts):
        """Unpack parts from payload"""

        for num_part in iter_range(expected_parts):
            hdr = payload.read(cls.header_size)
            try:
                part_header = PartHeader(*cls.header_struct.unpack(hdr))
            except struct.error:
                raise InterfaceError("No valid part header")

            if part_header.payload_size % 8 != 0:
                part_payload_size = part_header.payload_size + 8 - (part_header.payload_size % 8)
            else:
                part_payload_size = part_header.payload_size
            pl = payload.read(part_payload_size)
            part_payload = io.BytesIO(pl)
            try:
                _PartClass = PART_MAPPING[part_header.part_kind]
            except KeyError:
                raise InterfaceError("Unknown part kind %s" % part_header.part_kind)

            debug('%s (%d/%d): %s', _PartClass.__name__, num_part+1, expected_parts, str(part_header))
            debug('Read %d bytes payload for part %d', part_payload_size, num_part + 1)

            init_arguments = _PartClass.unpack_data(part_header.argument_count, part_payload)
            debug('Part data: %s', init_arguments)
            part = _PartClass(*init_arguments)
            part.header = part_header
            part.attribute = part_header.part_attributes
            part.source = 'server'
            if pyhdb.tracing:
                part.trace_header = humanhexlify(hdr[:part_header.payload_size])
                part.trace_payload = humanhexlify(pl, 30)
            yield part