docs: 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