Fix return type from schedule() with a list of one entry (#7885)

This commit fixes the return type when calling schedule() with a list of
a single entry. Previously this would return a single Schedule object
instead of the expected list return. A similar issue was already fixed
in transpile() in #5298 but schedule still had the incorrect behavior.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/compiler/scheduler.py

@@ -65,6 +65,7 @@ def schedule(
     Raises:
         QiskitError: If ``inst_map`` and ``meas_map`` are not passed and ``backend`` is not passed
     """
+    arg_circuits_list = isinstance(circuits, list)
     start_time = time()
     if backend and getattr(backend, "version", 0) > 1:
         if inst_map is None:
@@ -100,4 +101,7 @@ def schedule(
     schedules = [schedule_circuit(circuit, schedule_config, method) for circuit in circuits]
     end_time = time()
     _log_schedule_time(start_time, end_time)
-    return schedules[0] if len(schedules) == 1 else schedules
+    if arg_circuits_list:
+        return schedules
+    else:
+        return schedules[0]

releasenotes/notes/fix-list-input-schedule-14fc48895a061735.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :class:`~qiskit.compiler.schedule` function where
+    previously passing in a ``list`` of :class:`~qiskit.circuit.QuantumCircuit`
+    objects with a single entry would incorrectly return a single
+    :class:`~.Schedule` object instead of the expected ``list`` return type.

test/python/scheduler/test_basic_scheduler.py

@@ -82,6 +82,16 @@ class TestBasicSchedule(QiskitTestCase):
             self.assertEqual(actual[0], expected[0])
             self.assertEqual(actual[1], expected[1])
 
+    def test_single_circuit_list_schedule(self):
+        """Test that passing a single circuit list to schedule() returns a list."""
+        q = QuantumRegister(2)
+        c = ClassicalRegister(2)
+        qc = QuantumCircuit(q, c)
+        sched = schedule([qc], self.backend, method="alap")
+        expected = Schedule()
+        self.assertIsInstance(sched, list)
+        self.assertEqual(sched[0].instructions, expected.instructions)
+
     def test_alap_with_barriers(self):
         """Test that ALAP respects barriers on new qubits."""
         q = QuantumRegister(2)