Fix for "Add Operator Settings #6650" (#6670)

* Fix for "Add Operator Settings #6650"

* Fix lint

qiskit/opflow/gradients/gradient.py

@@ -189,7 +189,8 @@ class Gradient(GradientBase):
         elif isinstance(operator, ListOp):
             grad_ops = [self.get_gradient(op, param) for op in operator.oplist]
 
-            if operator._combo_fn is None:  # If using default
+            # pylint: disable=comparison-with-callable
+            if operator.combo_fn == ListOp.default_combo_fn:  # If using default
                 return ListOp(oplist=grad_ops)
             elif isinstance(operator, SummedOp):
                 return SummedOp(oplist=[grad for grad in grad_ops if grad != ~Zero @ One]).reduce()

qiskit/opflow/list_ops/list_op.py

@@ -14,7 +14,7 @@
 
 from functools import reduce
 from numbers import Number
-from typing import Callable, Dict, Iterator, List, Optional, Set, Sequence, Union, cast
+from typing import Any, Callable, Dict, Iterator, List, Optional, Set, Sequence, Union, cast
 
 import numpy as np
 from scipy.sparse import spmatrix
@@ -122,6 +122,11 @@ class ListOp(OperatorBase):
         """
         return self._oplist
 
+    @staticmethod
+    def default_combo_fn(x: Any) -> Any:
+        """ListOp default combo function i.e. lambda x: x"""
+        return x
+
     @property
     def combo_fn(self) -> Callable:
         """The function defining how to combine ``oplist`` (or Numbers, or NumPy arrays) to
@@ -132,7 +137,7 @@ class ListOp(OperatorBase):
             The combination function.
         """
         if self._combo_fn is None:
-            return lambda x: x
+            return ListOp.default_combo_fn
         return self._combo_fn
 
     @property