Make ListOp.num_qubits check all ops for same num_qubits

qiskit/aqua/operators/list_ops/list_op.py

@@ -129,7 +129,10 @@ def primitive_strings(self) -> Set[str]:
 
     @property
     def num_qubits(self) -> int:
-        return self.oplist[0].num_qubits
+        num_qubits0 = self.oplist[0].num_qubits
+        if not all(num_qubits0 == op.num_qubits for op in self.oplist):
+            raise ValueError('Operators in ListOp have differing numbers of qubits.')
+        return num_qubits0
 
     def add(self, other: OperatorBase) -> OperatorBase:
         if self == other:

releasenotes/notes/listop-check-num-qubits-ce3a02fcc862771a.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Make ListOp.num_qubits check that all ops in list have the same num_qubits
+    Previously, the number of qubits in the first operator in the ListOp
+    was returned. With this change, an additional check is made that all
+    other operators also have the same number of qubits.

test/aqua/operators/test_op_construction.py

@@ -508,6 +508,24 @@ def test_list_op_parameters(self):
         self.assertEqual(list_op.parameters, set(params))
 
 
+class TestOpMethods(QiskitAquaTestCase):
+    """Basic method tests."""
+
+    def test_listop_num_qubits(self):
+        """Test that ListOp.num_qubits checks that all operators have the same number of qubits."""
+        op = ListOp([X ^ Y, Y ^ Z])
+        with self.subTest('All operators have the same numbers of qubits'):
+            self.assertEqual(op.num_qubits, 2)
+
+        op = ListOp([X ^ Y, Y])
+        with self.subTest('Operators have different numbers of qubits'):
+            with self.assertRaises(ValueError):
+                op.num_qubits  # pylint: disable=pointless-statement
+
+            with self.assertRaises(ValueError):
+                X @ op  # pylint: disable=pointless-statement
+
+
 class TestListOpComboFn(QiskitAquaTestCase):
     """Test combo fn is propagated."""