TPBGroupedWeightedPauliOperator multiplication fix

CHANGELOG.md

@@ -58,6 +58,7 @@ Fixed
 -   Supported constant values on the left-hand side of constraints and variables on the right-hand
     side of constraints for the DOcplex translator (#750)
 -   WeightedPauliOperator constructor simplification bug (#891)
+-   TPBGroupedWeightedPauliOperator multiplication (#915)
 
 [0.6.6](https://github.com/Qiskit/qiskit-aqua/compare/0.6.5...0.6.6) - 2020-04-16
 =================================================================================

qiskit/aqua/operators/tpb_grouped_weighted_pauli_operator.py

@@ -231,21 +231,21 @@ def _add_or_sub(self, other, operation, copy=True):
         """
         # perform add or sub in paulis and then re-group it again
         ret_op = super()._add_or_sub(other, operation, copy)
-        # pylint: disable=no-member
-        ret_op = ret_op._grouping_func(ret_op, **self._kwargs)
+        ret_op = self._grouping_func(ret_op, **self._kwargs)
         return ret_op
 
     def multiply(self, other):
         """
         Perform self * other.
 
+        Note: Grouping of the new operator, will be as per self's grouping.
+
         Args:
             other (TPBGroupedWeightedPauliOperator): an operator
 
         Returns:
             TPBGroupedWeightedPauliOperator: the multiplied operator
         """
         ret_op = super().multiply(other)
-        # pylint: disable=no-member
-        ret_op = ret_op._grouping_func(ret_op, **self._kwargs)
+        ret_op = self._grouping_func(ret_op, **self._kwargs)
         return ret_op