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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 _retrieve_assets(self, sids, asset_tbl, asset_type):
 """ Internal function for loading assets from a table. This should be the only method of `AssetFinder` that writes Assets into self._asset_cache. Parameters --------- sids : iterable of int Asset ids to look up. asset_tbl : sqlalchemy.Table Table from which to query assets. asset_type : type Type of asset to be constructed. Returns ------- assets : dict[int -> Asset] Dict mapping requested sids to the retrieved assets. """

        # Fastpath for empty request.
        if not sids:
            return {}

        cache = self._asset_cache
        hits = {}

        querying_equities = issubclass(asset_type, Equity)
        filter_kwargs = (
            _filter_equity_kwargs
            if querying_equities else
            _filter_future_kwargs
        )

        rows = self._retrieve_asset_dicts(sids, asset_tbl, querying_equities)
        for row in rows:
            sid = row['sid']
            asset = asset_type(**filter_kwargs(row))
            hits[sid] = cache[sid] = asset

        # If we get here, it means something in our code thought that a
        # particular sid was an equity/future and called this function with a
        # concrete type, but we couldn't actually resolve the asset.  This is
        # an error in our code, not a user-input error.
        misses = tuple(set(sids) - viewkeys(hits))
        if misses:
            if querying_equities:
                raise EquitiesNotFound(sids=misses)
            else:
                raise FutureContractsNotFound(sids=misses)
        return hits