Retrieve asset types for a list of sids.

        Parameters
        ----------
        sids : list[int]

        Returns
        -------
        types : dict[sid -> str or None]
            Asset types for the provided sids.

def lookup_asset_types(self, sids):
        """
        Retrieve asset types for a list of sids.

        Parameters
        ----------
        sids : list[int]

        Returns
        -------
        types : dict[sid -> str or None]
            Asset types for the provided sids.
        """
        found = {}
        missing = set()

        for sid in sids:
            try:
                found[sid] = self._asset_type_cache[sid]
            except KeyError:
                missing.add(sid)

        if not missing:
            return found

        router_cols = self.asset_router.c

        for assets in group_into_chunks(missing):
            query = sa.select((router_cols.sid, router_cols.asset_type)).where(
                self.asset_router.c.sid.in_(map(int, assets))
            )
            for sid, type_ in query.execute().fetchall():
                missing.remove(sid)
                found[sid] = self._asset_type_cache[sid] = type_

            for sid in missing:
                found[sid] = self._asset_type_cache[sid] = None

        return found