Bugfix for split_non_commuting when tape measurements is empty

doc/releases/changelog-dev.md

@@ -115,6 +115,7 @@
 * `qml.transforms.split_non_commuting` can now handle circuits containing measurements of multi-term observables.
   [(#5729)](https://github.com/PennyLaneAI/pennylane/pull/5729)
   [(#5853)](https://github.com/PennyLaneAI/pennylane/pull/5838)
+  [(#5869)](https://github.com/PennyLaneAI/pennylane/pull/5869)
 
 * The qchem module has dedicated functions for calling `pyscf` and `openfermion` backends.
   [(#5553)](https://github.com/PennyLaneAI/pennylane/pull/5553)

pennylane/transforms/split_non_commuting.py

@@ -28,6 +28,13 @@
 from pennylane.typing import Result, ResultBatch
 
 
+def null_postprocessing(results):
+    """A postprocessing function returned by a transform that only converts the batch of results
+    into a result for a single ``QuantumTape``.
+    """
+    return results[0]
+
+
 @transform
 def split_non_commuting(
     tape: qml.tape.QuantumScript,
@@ -243,6 +250,8 @@ def circuit(x):
         [[expval(X(0)), probs(wires=[1])], [probs(wires=[0, 1])]]
 
     """
+    if len(tape.measurements) == 0:
+        return [tape], null_postprocessing
 
     # Special case for a single measurement of a Sum or Hamiltonian, in which case
     # the grouping information can be computed and cached in the observable.

tests/transforms/test_split_non_commuting.py

@@ -568,6 +568,17 @@ def test_tape_with_non_pauli_obs(self, non_pauli_obs):
             fn([[0.1, 0.2], [0.3, 0.6], 0.4, 0.5, 0.7]), [0.01, 0.06, 0.06, 0.16, 0.25, 0.36, 0.49]
         )
 
+    @pytest.mark.parametrize("grouping_strategy", [None, "default", "qwc", "wires"])
+    def test_no_measurements(self, grouping_strategy):
+        """Test that if the tape contains no measurements, the transform doesn't
+        modify it"""
+
+        tape = qml.tape.QuantumScript([qml.X(0)])
+        tapes, post_processing_fn = split_non_commuting(tape, grouping_strategy=grouping_strategy)
+        assert len(tapes) == 1
+        assert tapes[0] == tape
+        assert post_processing_fn(tapes) == tape
+
 
 class TestIntegration:
     """Tests the ``split_non_commuting`` transform performed on a QNode"""