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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 attach_db(self, db):
 """ Attach a gffutils.FeatureDB for access to features. Useful if you want to attach a db after this instance has already been created. Parameters db : gffutils.FeatureDB """

        if db is not None:
            if isinstance(db, basestring):
                db = gffutils.FeatureDB(db)
            if not isinstance(db, gffutils.FeatureDB):
                raise ValueError(
                    "`db` must be a filename or a gffutils.FeatureDB")
        self._kwargs['db'] = db
        self.db = db