Bug Fix for missing qubit property (#11880)

(cherry picked from commit c566b3f)

qiskit/providers/backend_compat.py

@@ -162,15 +162,22 @@ def _get_value(prop_dict, prop_name):
         faulty_qubits = {
             q for q in range(configuration.num_qubits) if not properties.is_qubit_operational(q)
         }
-        qubit_properties = [
-            QubitProperties(
-                t1=properties.qubit_property(qubit_idx)["T1"][0],
-                t2=properties.qubit_property(qubit_idx)["T2"][0],
-                frequency=properties.qubit_property(qubit_idx)["frequency"][0],
-            )
-            for qubit_idx in range(0, configuration.num_qubits)
-        ]
 
+        qubit_properties = []
+        for qi in range(0, configuration.num_qubits):
+            # TODO faulty qubit handling might be needed since
+            #  faulty qubit reporting qubit properties doesn't make sense.
+            try:
+                prop_dict = properties.qubit_property(qubit=qi)
+            except KeyError:
+                continue
+            qubit_properties.append(
+                QubitProperties(
+                    t1=prop_dict.get("T1", (None, None))[0],
+                    t2=prop_dict.get("T2", (None, None))[0],
+                    frequency=prop_dict.get("frequency", (None, None))[0],
+                )
+            )
         in_data["qubit_properties"] = qubit_properties
 
         for name in all_instructions:

releasenotes/notes/fix-missing-qubit-properties-35137aa5250d9368.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    A bug that crashes the :func:`.convert_to_target` function when qubit properties
+    (either T1, T2 or frequency) are missing was fixed.
+    The missing property values in :class:`.QubitProperties` are filled with ``None``.

test/python/providers/faulty_backends.py

@@ -34,6 +34,18 @@ def properties(self):
         return BackendProperties.from_dict(props)
 
 
+class Fake7QV1MissingQ1Property(Fake7QPulseV1):
+    """A fake 7 qubit backend, with a missing T1 property in q1."""
+
+    def properties(self):
+        """Returns a snapshot of device properties as recorded on 8/30/19.
+        Remove property from qubit 1.
+        """
+        props = super().properties().to_dict()
+        props["qubits"][1] = filter(lambda q: q["name"] != "T1", props["qubits"][1])
+        return BackendProperties.from_dict(props)
+
+
 class Fake7QV1FaultyCX01CX10(Fake7QPulseV1):
     """A fake 5 qubit backend, with faulty CX(Q1, Q0) and CX(Q0, Q1)
     0 (↔) 1 ↔ 3 ↔ 4

test/python/providers/test_faulty_backend.py

@@ -17,6 +17,7 @@
 from .faulty_backends import (
     Fake7QV1FaultyCX01CX10,
     Fake7QV1FaultyQ1,
+    Fake7QV1MissingQ1Property,
     Fake7QV1FaultyCX13CX31,
 )
 
@@ -35,7 +36,7 @@ def test_faulty_qubits(self):
         """Test faulty_qubits method."""
         self.assertEqual(self.backend.properties().faulty_qubits(), [1])
 
-    def test_convert_to_target(self):
+    def test_convert_to_target_with_filter(self):
         """Test converting legacy data structure to V2 target model with faulty qubits.
 
         Measure and Delay are automatically added to the output Target
@@ -44,31 +45,40 @@ def test_convert_to_target(self):
         """
 
         # Filter out faulty Q1
-        target_with_filter = convert_to_target(
+        target = convert_to_target(
             configuration=self.backend.configuration(),
             properties=self.backend.properties(),
             add_delay=True,
             filter_faulty=True,
         )
-        self.assertFalse(
-            target_with_filter.instruction_supported(operation_name="measure", qargs=(1,))
-        )
-        self.assertFalse(
-            target_with_filter.instruction_supported(operation_name="delay", qargs=(1,))
-        )
+        self.assertFalse(target.instruction_supported(operation_name="measure", qargs=(1,)))
+        self.assertFalse(target.instruction_supported(operation_name="delay", qargs=(1,)))
+
+    def test_convert_to_target_without_filter(self):
+        """Test converting legacy data structure to V2 target model with faulty qubits."""
 
         # Include faulty Q1 even though data could be incomplete
-        target_without_filter = convert_to_target(
+        target = convert_to_target(
             configuration=self.backend.configuration(),
             properties=self.backend.properties(),
             add_delay=True,
             filter_faulty=False,
         )
-        self.assertTrue(
-            target_without_filter.instruction_supported(operation_name="measure", qargs=(1,))
+        self.assertTrue(target.instruction_supported(operation_name="measure", qargs=(1,)))
+        self.assertTrue(target.instruction_supported(operation_name="delay", qargs=(1,)))
+
+        # Properties are preserved
+        self.assertEqual(
+            target.qubit_properties[1].t1,
+            self.backend.properties().t1(1),
+        )
+        self.assertEqual(
+            target.qubit_properties[1].t2,
+            self.backend.properties().t2(1),
         )
-        self.assertTrue(
-            target_without_filter.instruction_supported(operation_name="delay", qargs=(1,))
+        self.assertEqual(
+            target.qubit_properties[1].frequency,
+            self.backend.properties().frequency(1),
         )
 
 
@@ -108,3 +118,30 @@ def test_faulty_gates(self):
         self.assertEqual(len(gates), 2)
         self.assertEqual([gate.gate for gate in gates], ["cx", "cx"])
         self.assertEqual(sorted(gate.qubits for gate in gates), [[0, 1], [1, 0]])
+
+
+class MissingPropertyQubitBackendTestCase(QiskitTestCase):
+    """Test operational-related methods of backend.properties() with Fake7QV1MissingQ1Property,
+    which is like Fake7QV1 but with Q1 with missing T1 property."""
+
+    backend = Fake7QV1MissingQ1Property()
+
+    def test_convert_to_target(self):
+        """Test converting legacy data structure to V2 target model with missing qubit property."""
+
+        target = convert_to_target(
+            configuration=self.backend.configuration(),
+            properties=self.backend.properties(),
+            add_delay=True,
+            filter_faulty=True,
+        )
+
+        self.assertIsNone(target.qubit_properties[1].t1)
+        self.assertEqual(
+            target.qubit_properties[1].t2,
+            self.backend.properties().t2(1),
+        )
+        self.assertEqual(
+            target.qubit_properties[1].frequency,
+            self.backend.properties().frequency(1),
+        )