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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _parse_block(self):
 """Parse metadata and rows from the block only once."""

        if self._iter_rows is not None:
            return

        rows = _avro_rows(self._block, self._avro_schema)
        self._num_items = self._block.avro_rows.row_count
        self._remaining = self._block.avro_rows.row_count
        self._iter_rows = iter(rows)