Gracefully handle batch execution of no circuits

CHANGELOG.md

@@ -4,31 +4,31 @@
 
 ### Improvements 🛠
 
-* The UI for passing parameters to a ``qfunc`` generated when loading a Qiskit ``QuantumCircuit``
+* The UI for passing parameters to a `qfunc` generated when loading a Qiskit `QuantumCircuit`
   into PennyLane is updated to allow passing parameters as args or kwargs, rather than as
   a dictionary. The old dictionary UI continues to be supported.
   [(#406)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/406)
   [(#428)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/428)
 
-* Measurement operations are now added to the PennyLane template when a ``QuantumCircuit``
+* Measurement operations are now added to the PennyLane template when a `QuantumCircuit`
   is converted using `load`. Additionally, one can override any existing terminal
   measurements by providing a list of PennyLane
   `measurements <https://docs.pennylane.ai/en/stable/introduction/measurements.html>`_ themselves.
   [(#405)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/405)
   [(#466)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/466)
 
 * Added the support for converting conditional operations based on mid-circuit measurements and
-  two of the ``ControlFlowOp`` operations - ``IfElseOp`` and ``SwitchCaseOp`` when converting
-  a ``QuantumCircuit`` using `load`.
+  two of the `ControlFlowOp` operations - `IfElseOp` and `SwitchCaseOp` when converting
+  a `QuantumCircuit` using `load`.
   [(#417)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/417)
   [(#465)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/465)
 
-* Qiskit's classical ``Expr`` conditionals can also be used with the supported
-  ``ControlFlowOp`` operations.
+* Qiskit's classical `Expr` conditionals can also be used with the supported
+  `ControlFlowOp` operations.
   [(#432)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/432)
 
 * Added conversion support for more Qiskit gates to native PennyLane operations -
-  ``Barrier``, ``CYGate``, ``CHGate``, ``CPhase``, ``CCZGate``, ``ECRGate``, and ``GlobalPhaseGate``.
+  `Barrier`, `CYGate`, `CHGate`, `CPhase`, `CCZGate`, `ECRGate`, and `GlobalPhaseGate`.
   [(#449)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/449)
 
 * Added the ability to convert a Qiskit `SparsePauliOp` instance into a PennyLane `Operator`.
@@ -49,6 +49,9 @@
 
 ### Bug fixes 🐛
 
+* `QiskitDevice.batch_execute()` now gracefully handles empty lists of circuits.
+  [(#459)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/459)
+
 * It is now possible to compute the gradient of a circuit with `ParameterVector` elements.
   [(#458)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/458)
 
@@ -477,7 +480,7 @@ Christina Lee, Antal Száva
 
 ### Improvements
 
-* The plugin can now load Qiskit circuits with more complicated ``ParameterExpression`` variables.
+* The plugin can now load Qiskit circuits with more complicated `ParameterExpression` variables.
   [(#139)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/139)
 
 ### Contributors
@@ -629,7 +632,7 @@ Maria Schuld, Antal Száva
 
 ### Improvements
 
-* Ported the ``QiskitDevice`` class to inherit from the ``QubitDevice`` class
+* Ported the `QiskitDevice` class to inherit from the `QubitDevice` class
   defined in PennyLane to use unified qubit operations and ease development.
   [(#83)](https://github.com/XanaduAI/pennylane-qiskit/pull/83)
 

pennylane_qiskit/qiskit_device.py

@@ -496,6 +496,10 @@ def batch_execute(self, circuits, timeout: int = None):
 
         compiled_circuits = self.compile_circuits(circuits)
 
+        if not compiled_circuits:
+            # At least one circuit must always be provided to the backend.
+            return []
+
         # Send the batch of circuit objects using backend.run
         self._current_job = self.backend.run(compiled_circuits, shots=self.shots, **self.run_args)
 

tests/test_qiskit_device.py

@@ -162,6 +162,16 @@ def test_result(self, device, tol):
         assert isinstance(res[1], np.ndarray)
         assert np.allclose(res[1], tape2_expected, atol=0)
 
+    def test_result_no_tapes(self, device):
+        """Tests that the result is correct when there are no tapes to execute."""
+        dev = device(2)
+        res = dev.batch_execute([])
+
+        # We're calling device methods directly, need to reset before the next
+        # execution
+        dev.reset()
+        assert not res
+
     def test_result_empty_tape(self, device, tol):
         """Tests that the result has the correct shape and entry types for
         empty tapes."""