code: def _make_sid_selector(self, assets):
          
          assets = np.array(assets)
          sid_selector = self.sids.searchsorted(assets)
          unknown = np.in1d(assets, self.sids, invert=True)
          sid_selector[unknown] = -1
          return sid_selector
docs: Build an indexer mapping ``self.sids`` to ``assets``.
  
          Parameters
          ----------
          assets : list[int]
              List of assets requested by a caller of ``load_raw_arrays``.
  
          Returns
          -------
          index : np.array[int64]
              Index array containing the index in ``self.sids`` for each location
              in ``assets``. Entries in ``assets`` for which we don't have a sid
              will contain -1. It is caller's responsibility to handle these
              values correctly.