Deprecate QauntumScript.output_dim (#6577)

**Context:**
This property is now slightly wrong, and is only used by
`parameter_shift_cv.py`.
**Description of the Change:**
- Deprecated the property in the original src,
`qml.tape.qscript.py::QuantumScript`
 - DeprecateWarning the tests in `test_qscript.py`
 - Removed the tests in `test_tape.py` and `test_legacy_device.py`
- Modified the original method and moved to
`qml.gradients.parameter_shift_cv.py` which is the only module using
this property

**Benefits:**
 - Less redundancy

**Possible Drawbacks:**
Potential uncovered usecases of this property inside or outside PL

**Related GitHub Issues:**

**Related Shortcut Stories:**
[sc-74594]

---------

Co-authored-by: Christina Lee <[email protected]>
Co-authored-by: lillian542 <[email protected]>
Co-authored-by: Alex Preciado <[email protected]>

doc/development/deprecations.rst

@@ -9,6 +9,11 @@ deprecations are listed below.
 Pending deprecations
 --------------------
 
+* The ``output_dim`` property of ``qml.tape.QuantumScript`` has been deprecated. Instead, use method ``shape`` of ``QuantumScript`` or ``MeasurementProcess`` to get the same information.
+
+  - Deprecated in v0.40
+  - Will be removed in v0.41
+
 * The ``QNode.get_best_method`` and ``QNode.best_method_str`` methods have been deprecated. 
   Instead, use the ``qml.workflow.get_best_diff_method``. 
 

doc/releases/changelog-dev.md

@@ -98,6 +98,11 @@
 
 <h3>Deprecations 👋</h3>
 
+* The `output_dim` property of `qml.tape.QuantumScript` has been deprecated. 
+Instead, use method `shape` of `QuantumScript` or `MeasurementProcess` to get the 
+same information.
+  [(#6577)](https://github.com/PennyLaneAI/pennylane/pull/6577)
+
 * The `QNode.get_best_method` and `QNode.best_method_str` methods have been deprecated. 
   Instead, use the `qml.workflow.get_best_diff_method` function.
   [(#6418)](https://github.com/PennyLaneAI/pennylane/pull/6418)

pennylane/gradients/parameter_shift_cv.py

@@ -450,7 +450,7 @@ def processing_fn(results):
         start = 0
 
         if not results:
-            results = [np.squeeze(np.zeros([tape.output_dim]))]
+            results = [np.array(0.0)]
 
         interface = qml.math.get_interface(results[0])
         iterator = enumerate(zip(shapes, gradient_values, obs_indices))
@@ -726,7 +726,7 @@ def _update(data):
         fns.append(data[1])
 
     if all(g == "0" for g in method_map.values()):
-        return [], lambda _: np.zeros([tape.output_dim, len(tape.trainable_params)])
+        return [], lambda _: np.zeros([1, len(tape.trainable_params)])
 
     var_present = any(isinstance(m, VarianceMP) for m in tape.measurements)
 

pennylane/tape/qscript.py

@@ -324,7 +324,23 @@ def batch_size(self) -> Optional[int]:
 
     @property
     def output_dim(self) -> int:
-        """The (inferred) output dimension of the quantum script."""
+        """The (inferred) output dimension of the quantum script.
+
+        .. warning::
+
+            ``QuantumScript.output_dim`` is being deprecated. Instead, considering
+            using method ``shape`` of ``QuantumScript`` or ``MeasurementProcess``
+            to get the same information. See ``qml.gradients.parameter_shift_cv.py::_get_output_dim``
+            for an example.
+
+        """
+        # pylint: disable=import-outside-toplevel
+        import warnings
+
+        warnings.warn(
+            "The 'output_dim' property is deprecated and will be removed in version 0.41",
+            qml.PennyLaneDeprecationWarning,
+        )
         if self._output_dim is None:
             self._update_output_dim()  # this will set _batch_size if it isn't already
         return self._output_dim

tests/devices/test_legacy_device.py

@@ -652,7 +652,6 @@ def test_default_expand_with_initial_state(self, op, decomp):
         assert new_tape.shots is tape.shots
         assert new_tape.wires == tape.wires
         assert new_tape.batch_size == tape.batch_size
-        assert new_tape.output_dim == tape.output_dim
 
     def test_default_expand_fn_with_invalid_op(self, mock_device_supporting_paulis, recwarn):
         """Test that default_expand_fn works with an invalid op and some measurement."""

tests/tape/test_qscript.py

@@ -332,7 +332,11 @@ def test_error_inconsistent_batch_sizes(self, x, rot, y):
     def test_update_output_dim(self, m, output_dim, ops, factor):
         """Test setting the output_dim property."""
         qs = QuantumScript(ops, m)
-        assert qs.output_dim == output_dim * factor
+
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert qs.output_dim == output_dim * factor
 
     def test_lazy_batch_size_and_output_dim(self):
         """Test that batch_size and output_dim are computed lazily."""
@@ -347,7 +351,11 @@ def test_lazy_batch_size_and_output_dim(self):
         # now evaluate it
         assert qs.batch_size == 2
         assert qs._output_dim is None  # setting batch_size didn't set output_dim
-        assert qs.output_dim == 2
+
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert qs.output_dim == 2
         copied = qs.copy()
         assert qs._batch_size == 2
         assert qs._output_dim == 2
@@ -361,7 +369,10 @@ def test_lazy_setting_output_dim_sets_batch_size(self):
         assert qs._batch_size is _UNSET_BATCH_SIZE
         assert qs._output_dim is None
 
-        assert qs.output_dim == 2  # getting this sets both _output_dim and _batch_size
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert qs.output_dim == 2  # getting this sets both _output_dim and _batch_size
         assert qs._output_dim == 2
         assert qs._batch_size == 2
 
@@ -573,7 +584,10 @@ def test_shallow_copy(self):
         assert qs.shots is copied_qs.shots
 
         # check that the output dim is identical
-        assert qs.output_dim == copied_qs.output_dim
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert qs.output_dim == copied_qs.output_dim
 
     # pylint: disable=unnecessary-lambda
     @pytest.mark.parametrize(
@@ -608,7 +622,10 @@ def test_shallow_copy_with_operations(self, copy_fn):
         assert qs.shots is copied_qs.shots
 
         # check that the output dim is identical
-        assert qs.output_dim == copied_qs.output_dim
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert qs.output_dim == copied_qs.output_dim
 
     def test_deep_copy(self):
         """Test that deep copying a tape works, and copies all constituent data except parameters"""
@@ -628,7 +645,10 @@ def test_deep_copy(self):
         assert copied_qs.shots is qs.shots
 
         # check that the output dim is identical
-        assert qs.output_dim == copied_qs.output_dim
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert qs.output_dim == copied_qs.output_dim
 
         # The underlying operation data has also been copied
         assert copied_qs.operations[0].wires is not qs.operations[0].wires
@@ -743,7 +763,10 @@ def test_cached_properties_when_updating_operations(self):
         tape = QuantumScript(ops, measurements=[qml.counts()], shots=2500, trainable_params=[1])
 
         assert tape.batch_size == 2
-        assert tape.output_dim == 2
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert tape.output_dim == 2
         assert tape.trainable_params == [1]
 
         new_ops = [qml.RX([1.2, 2.3, 3.4], 0)]
@@ -753,7 +776,10 @@ def test_cached_properties_when_updating_operations(self):
         assert new_tape.operations == new_ops
 
         assert new_tape.batch_size == 3
-        assert new_tape.output_dim == 3
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert new_tape.output_dim == 3
         assert new_tape.trainable_params == [0]
 
     def test_cached_properties_when_updating_measurements(self):
@@ -771,7 +797,10 @@ def test_cached_properties_when_updating_measurements(self):
 
         assert tape.obs_sharing_wires == []
         assert tape.obs_sharing_wires_id == []
-        assert tape.output_dim == 2
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert tape.output_dim == 2
         assert tape.trainable_params == [1]
 
         new_measurements = [qml.expval(qml.X(0)), qml.var(qml.Y(0))]
@@ -780,7 +809,10 @@ def test_cached_properties_when_updating_measurements(self):
         assert tape.measurements == measurements
         assert new_tape.measurements == new_measurements
 
-        assert new_tape.output_dim == 4
+        with pytest.warns(
+            qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+        ):
+            assert new_tape.output_dim == 4
         assert new_tape.obs_sharing_wires == [qml.X(0), qml.Y(0)]
         assert new_tape.obs_sharing_wires_id == [0, 1]
         assert new_tape.trainable_params == [0, 1]
@@ -1702,3 +1734,14 @@ def test_jax_pytree_integration(qscript_type):
     assert data[3] == 3.4
     assert data[4] == 2.0
     assert qml.math.allclose(data[5], eye_mat)
+
+
+@pytest.mark.parametrize("qscript_type", (QuantumScript, qml.tape.QuantumTape))
+@pytest.mark.parametrize("shots", [None, 1, 10])
+def test_output_dim_is_deprecated(qscript_type, shots):
+    """Test that the output_dim property is deprecated."""
+    with pytest.warns(
+        qml.PennyLaneDeprecationWarning, match="The 'output_dim' property is deprecated"
+    ):
+        qscript = qscript_type([], [], shots=shots)
+        _ = qscript.output_dim

tests/tape/test_tape.py

@@ -85,11 +85,9 @@ def test_qubit_queuing(self, make_tape):
         assert len(tape.queue) == 6
         assert tape.operations == ops
         assert tape.observables == obs
-        assert tape.output_dim == 5
         assert tape.batch_size is None
 
         assert tape.wires == qml.wires.Wires([0, "a", 4])
-        assert tape._output_dim == len(obs[0].wires) + 2 ** len(obs[1].wires)
 
     def test_observable_processing(self, make_tape):
         """Test that observables are processed correctly"""
@@ -202,7 +200,6 @@ def test_tensor_process_queuing(self):
         assert not tape.operations
         assert tape.measurements == [D]
         assert tape.observables == [C]
-        assert tape.output_dim == 1
         assert tape.batch_size is None
 
     def test_multiple_contexts(self):
@@ -225,7 +222,6 @@ def test_multiple_contexts(self):
         assert len(tape.queue) == 4
         assert tape.operations == ops
         assert tape.observables == obs
-        assert tape.output_dim == 5
         assert tape.batch_size is None
 
         assert not any(qml.equal(a, op) or qml.equal(b, op) for op in tape.operations)
@@ -1245,7 +1241,6 @@ def test_execute_parameters(self, tol):
             qml.CNOT(wires=[0, 1])
             qml.expval(qml.PauliZ(0) @ qml.PauliX(1))
 
-        assert tape.output_dim == 1
         assert tape.batch_size is None
 
         # test execution with no parameters
@@ -1284,8 +1279,6 @@ def test_single_expectation_value(self, tol):
             qml.CNOT(wires=[0, 1])
             qml.expval(qml.PauliZ(0) @ qml.PauliX(1))
 
-        assert tape.output_dim == 1
-
         res = dev.execute(tape)
         assert res.shape == ()
 
@@ -1306,8 +1299,6 @@ def test_multiple_expectation_values(self, tol):
             qml.expval(qml.PauliZ(0))
             qml.expval(qml.PauliX(1))
 
-        assert tape.output_dim == 2
-
         res = dev.execute(tape)
         assert isinstance(res, tuple)
         assert len(res) == 2
@@ -1329,8 +1320,6 @@ def test_var_expectation_values(self, tol):
             qml.expval(qml.PauliZ(0))
             qml.var(qml.PauliX(1))
 
-        assert tape.output_dim == 2
-
         res = dev.execute(tape)
         assert isinstance(res, tuple)
         assert len(res) == 2
@@ -1353,8 +1342,6 @@ def test_prob_expectation_values(self, tol):
             qml.expval(qml.PauliZ(0))
             qml.probs(wires=[0, 1])
 
-        assert tape.output_dim == 5
-
         res = dev.execute(tape)
 
         assert isinstance(res, tuple)
@@ -1461,8 +1448,6 @@ def test_single_output_value(self):
             qml.Beamsplitter(np.pi / 4, 0, wires=[0, 1])
             qml.expval(qml.NumberOperator(0))
 
-        assert tape.output_dim == 1
-
         res = dev.batch_execute([tape])[0]
         assert res.shape == ()
 
@@ -1497,9 +1482,6 @@ def test_shallow_copy(self):
         assert tape.wires == copied_tape.wires
         assert tape.data == copied_tape.data
 
-        # check that the output dim is identical
-        assert tape.output_dim == copied_tape.output_dim
-
     @pytest.mark.parametrize("copy_fn", [lambda tape: tape.copy(copy_operations=True), copy.copy])
     def test_shallow_copy_with_operations(self, copy_fn):
         """Test that shallow copying of a tape and operations allows
@@ -1528,9 +1510,6 @@ def test_shallow_copy_with_operations(self, copy_fn):
         assert tape.wires == copied_tape.wires
         assert tape.data == copied_tape.data
 
-        # check that the output dim is identical
-        assert tape.output_dim == copied_tape.output_dim
-
     def test_deep_copy(self):
         """Test that deep copying a tape works, and copies all constituent data except parameters"""
         with QuantumTape() as tape:
@@ -1548,9 +1527,6 @@ def test_deep_copy(self):
         assert all(o1 is not o2 for o1, o2 in zip(copied_tape.observables, tape.observables))
         assert all(m1 is not m2 for m1, m2 in zip(copied_tape.measurements, tape.measurements))
 
-        # check that the output dim is identical
-        assert tape.output_dim == copied_tape.output_dim
-
         # The underlying operation data has also been copied
         assert copied_tape.operations[0].wires is not tape.operations[0].wires