def _get_evaluated_constants(self, backend: str) -> List[Optional[ArrayType]]:
        """Retrieve or generate the cached list of constant operators (mixed
        in with None representing non-consts) and the remaining contraction
        list.
        """
        try:
            return self._evaluated_constants[backend]
        except KeyError:
            self.evaluate_constants(backend)
            return self._evaluated_constants[backend]
Retrieve or generate the cached list of constant operators (mixed
        in with None representing non-consts) and the remaining contraction
        list.
        
opt_einsum/contract.py