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

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