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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 open_final_ddb(self):
 """ Open the DDB file located in the output directory of the flow. Return: :class:`DdbFile` object, None if file could not be found or file is not readable. """

        ddb_path = self.outdir.has_abiext("DDB")
        if not ddb_path:
            if self.status == self.S_OK:
                logger.critical("%s reached S_OK but didn't produce a GSR file in %s" % (self, self.outdir))
            return None

        from abipy.dfpt.ddb import DdbFile
        try:
            return DdbFile(ddb_path)
        except Exception as exc:
            logger.critical("Exception while reading DDB file at %s:\n%s" % (ddb_path, str(exc)))
            return None