transpile a list of circuits with a single element should return a list with a single element

qiskit/compiler/transpile.py

@@ -191,19 +191,19 @@ def callback_func(**kwargs):
         TranspilerError: in case of bad inputs to transpiler (like conflicting parameters)
             or errors in passes
     """
-    circuits = circuits if isinstance(circuits, list) else [circuits]
+    arg_circuits_list = isinstance(circuits, list)
+    circuits = circuits if arg_circuits_list else [circuits]
 
     # transpiling schedules is not supported yet.
     start_time = time()
     if all(isinstance(c, Schedule) for c in circuits):
         warnings.warn("Transpiling schedules is not supported yet.", UserWarning)
-        if len(circuits) == 1:
-            end_time = time()
-            _log_transpile_time(start_time, end_time)
-            return circuits[0]
         end_time = time()
         _log_transpile_time(start_time, end_time)
-        return circuits
+        if arg_circuits_list:
+            return circuits
+        else:
+            return circuits[0]
 
     if pass_manager is not None:
         _check_conflicting_argument(optimization_level=optimization_level, basis_gates=basis_gates,
@@ -242,13 +242,13 @@ def callback_func(**kwargs):
     # Transpile circuits in parallel
     circuits = parallel_map(_transpile_circuit, list(zip(circuits, transpile_args)))
 
-    if len(circuits) == 1:
-        end_time = time()
-        _log_transpile_time(start_time, end_time)
-        return circuits[0]
     end_time = time()
     _log_transpile_time(start_time, end_time)
-    return circuits
+
+    if arg_circuits_list:
+        return circuits
+    else:
+        return circuits[0]
 
 
 def _check_conflicting_argument(**kargs):

releasenotes/notes/transpile_singleton-61755c5cf1970111.yaml

@@ -0,0 +1,23 @@
+---
+fixes:
+  - |
+    When running :func:`qiskit.compiler.transpile` on a list of circuits with a
+    single element, the function used to return a circuit instead of a list. Now,
+    when :func:`qiskit.compiler.transpile` is called with a list, it will return a
+    list if when that list has a single element. See
+    `#5260 <https://github.com/Qiskit/qiskit-terra/issues/5260>`__.
+
+    .. code-block:: python
+
+      from qiskit import *
+
+      qc = QuantumCircuit(2)
+      qc.h(0)
+      qc.cx(0, 1)
+      qc.measure_all()
+
+      transpiled = transpile([qc])
+      print(type(transpiled), len(transpiled))
+
+    .. parsed-literal::
+     <class 'list'> 1

test/python/compiler/test_transpiler.py

@@ -257,6 +257,22 @@ def test_transpile_two(self):
         self.assertIsInstance(circuits[0], QuantumCircuit)
         self.assertIsInstance(circuits[1], QuantumCircuit)
 
+    def test_transpile_singleton(self):
+        """Test transpile a single-element list with a circuit.
+        See https://github.com/Qiskit/qiskit-terra/issues/5260"""
+        backend = BasicAer.get_backend('qasm_simulator')
+
+        qubit_reg = QuantumRegister(2)
+        clbit_reg = ClassicalRegister(2)
+        qc = QuantumCircuit(qubit_reg, clbit_reg, name="bell")
+        qc.h(qubit_reg[0])
+        qc.cx(qubit_reg[0], qubit_reg[1])
+        qc.measure(qubit_reg, clbit_reg)
+
+        circuits = transpile([qc], backend)
+        self.assertEqual(len(circuits), 1)
+        self.assertIsInstance(circuits[0], QuantumCircuit)
+
     def test_mapping_correction(self):
         """Test mapping works in previous failed case.
         """