Make ListOp.num_qubits check all ops for same num_qubits (qiskit-comm…

…unity#1189)

* Make ListOp.num_qubits check all ops for 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.

Closes qiskit-community#1086

* Add release note for listop-check-num-qubits

Co-authored-by: Manoel Marques <Manoel.Marques@ibm.com>

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."""