Remove output_dim property of qml.tape.QuantumScript (#6829)

**Context:** The property `output_dim` of `QuantumScript` has been deprecated in previous round, [sc-74594] #6577. Now we would love to remove it entirely. **Description of the Change:** - Remove property `output_dim` along with its tests, warnings, etc. - Remove the detached attribute `_output_dim` after previous removal. - Remove the detached private method `_update_output_dim` which used to be strictly only helping update property `output_dim`. **Benefits:** Cleaner codebase with less redundancies. **Possible Drawbacks:** Not known. Perhaps some nuances hidden in some uncovered cornercase tests. Anyways potential drawback won't affect any current feature in principle. **Related GitHub Issues:** [sc-82150] --------- Co-authored-by: lillian542 <[email protected]>
PennyLaneAI · Jan 15, 2025 · 3bec1cd · 3bec1cd
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diff --git a/doc/development/deprecations.rst b/doc/development/deprecations.rst
@@ -25,11 +25,6 @@ Pending deprecations
   - Deprecated in v0.40
   - Will be removed in v0.41
 
-* 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`` function. 
 
@@ -84,6 +79,11 @@ for details on how to port your legacy code to the new system. The following fun
 Completed deprecation cycles
 ----------------------------
 
+* The ``output_dim`` property of ``qml.tape.QuantumScript`` has been removed. Instead, use method ``shape`` of ``QuantumScript`` or ``MeasurementProcess`` to get the same information.
+
+  - Deprecated in v0.40
+  - Removed in v0.41
+
 * The ``qml.qsvt_legacy`` function has been removed.
   Instead, use ``qml.qsvt``. The new functionality takes an input polynomial instead of angles.
 

diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
@@ -11,6 +11,9 @@
 
 <h3>Breaking changes 💔</h3>
 
+* The `output_dim` property of `qml.tape.QuantumScript` has been removed. Instead, use method `shape` of `QuantumScript` or `MeasurementProcess` to get the same information.
+  [(#6829)](https://github.com/PennyLaneAI/pennylane/pull/6829)
+
 * Removed method `qsvt_legacy` along with its private helper `_qsp_to_qsvt`
   [(#6827)](https://github.com/PennyLaneAI/pennylane/pull/6827)
 

diff --git a/pennylane/devices/_legacy_device.py b/pennylane/devices/_legacy_device.py
@@ -93,7 +93,6 @@ def _local_tape_expand(tape, depth, stop_at):
 
     # Update circuit info
     new_tape._batch_size = tape._batch_size
-    new_tape._output_dim = tape._output_dim
     return new_tape
 
 

diff --git a/pennylane/tape/qscript.py b/pennylane/tape/qscript.py
@@ -25,7 +25,7 @@
 from typing import Any, Optional, TypeVar, Union
 
 import pennylane as qml
-from pennylane.measurements import MeasurementProcess, ProbabilityMP, StateMP
+from pennylane.measurements import MeasurementProcess
 from pennylane.measurements.shots import Shots, ShotsLike
 from pennylane.operation import _UNSET_BATCH_SIZE, Observable, Operation, Operator
 from pennylane.pytrees import register_pytree
@@ -183,7 +183,6 @@ def __init__(
         self._trainable_params = trainable_params
         self._graph = None
         self._specs = None
-        self._output_dim = None
         self._batch_size = _UNSET_BATCH_SIZE
 
         self._obs_sharing_wires = None
@@ -322,29 +321,6 @@ def batch_size(self) -> Optional[int]:
             self._update_batch_size()
         return self._batch_size
 
-    @property
-    def output_dim(self) -> int:
-        """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
-
     @property
     def diagonalizing_gates(self) -> list[Operation]:
         """Returns the gates that diagonalize the measured wires such that they
@@ -566,28 +542,6 @@ def _update_batch_size(self):
 
         self._batch_size = candidate
 
-    def _update_output_dim(self):
-        """Update the dimension of the output of the quantum script.
-
-        Sets:
-            self._output_dim (int): Size of the quantum script output (when flattened)
-
-        This method makes use of `self.batch_size`, so that `self._batch_size`
-        needs to be up to date when calling it.
-        Call `_update_batch_size` before `_update_output_dim`
-        """
-        self._output_dim = 0
-        for m in self.measurements:
-            # attempt to infer the output dimension
-            if isinstance(m, ProbabilityMP):
-                # TODO: what if we had a CV device here? Having the base as
-                # 2 would have to be swapped to the cutoff value
-                self._output_dim += 2 ** len(m.wires)
-            elif not isinstance(m, StateMP):
-                self._output_dim += 1
-        if self.batch_size:
-            self._output_dim *= self.batch_size
-
     # ========================================================
     # Parameter handling
     # ========================================================
@@ -984,9 +938,6 @@ def copy(self, copy_operations: bool = False, **update) -> "QuantumScript":
             # obs may change if measurements were updated
             new_qscript._obs_sharing_wires = self._obs_sharing_wires
             new_qscript._obs_sharing_wires_id = self._obs_sharing_wires_id
-        if not (update.get("measurements") or update.get("operations")):
-            # output_dim may change if either measurements or operations were updated
-            new_qscript._output_dim = self._output_dim
         return new_qscript
 
     def __copy__(self) -> "QuantumScript":

diff --git a/pennylane/tape/tape.py b/pennylane/tape/tape.py
@@ -294,7 +294,6 @@ def stop_at(obj):  # pylint: disable=unused-argument
 
     # Update circuit info
     new_tape._batch_size = tape._batch_size
-    new_tape._output_dim = tape._output_dim
     return new_tape
 
 
@@ -343,7 +342,6 @@ def expand_tape_state_prep(tape, skip_first=True):
 
     # Update circuit info
     new_tape._batch_size = tape._batch_size
-    new_tape._output_dim = tape._output_dim
     return new_tape
 
 

diff --git a/tests/tape/test_qscript.py b/tests/tape/test_qscript.py
@@ -318,63 +318,21 @@ def test_error_inconsistent_batch_sizes(self, x, rot, y):
         ):
             _ = tape.batch_size
 
-    @pytest.mark.parametrize(
-        "m, output_dim",
-        [
-            ([qml.expval(qml.PauliX(0))], 1),
-            ([qml.expval(qml.PauliX(0)), qml.var(qml.PauliY(1))], 2),
-            ([qml.probs(wires=(0, 1))], 4),
-            ([qml.state()], 0),
-            ([qml.probs((0, 1)), qml.expval(qml.PauliX(0))], 5),
-        ],
-    )
-    @pytest.mark.parametrize("ops, factor", [([], 1), ([qml.RX([1.2, 2.3, 3.4], wires=0)], 3)])
-    def test_update_output_dim(self, m, output_dim, ops, factor):
-        """Test setting the output_dim property."""
-        qs = QuantumScript(ops, m)
-
-        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."""
+    def test_lazy_batch_size(self):
+        """Test that batch_size is computed lazily."""
         qs = QuantumScript([qml.RX([1.1, 2.2], 0)], [qml.expval(qml.PauliZ(0))])
         copied = qs.copy()
         assert qs._batch_size is _UNSET_BATCH_SIZE
-        assert qs._output_dim is None
         # copying did not evaluate them either
         assert copied._batch_size is _UNSET_BATCH_SIZE
-        assert copied._output_dim is None
 
         # now evaluate it
         assert qs.batch_size == 2
-        assert qs._output_dim is None  # setting batch_size didn't set output_dim
 
-        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
         # copied tape has it pre-evaluated
         assert copied._batch_size == 2
-        assert copied._output_dim == 2
-
-    def test_lazy_setting_output_dim_sets_batch_size(self):
-        """Test that setting the output_dim also sets the batch_size."""
-        qs = QuantumScript([qml.RX([1.1, 2.2], 0)], [qml.expval(qml.PauliZ(0))])
-        assert qs._batch_size is _UNSET_BATCH_SIZE
-        assert qs._output_dim is None
-
-        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
 
 
 class TestIteration:
@@ -583,12 +541,6 @@ def test_shallow_copy(self):
         assert qs.data == copied_qs.data
         assert qs.shots is copied_qs.shots
 
-        # check that the output dim is identical
-        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(
         "copy_fn", [lambda tape: tape.copy(copy_operations=True), lambda tape: copy.copy(tape)]
@@ -621,12 +573,6 @@ def test_shallow_copy_with_operations(self, copy_fn):
         assert qs.data == copied_qs.data
         assert qs.shots is copied_qs.shots
 
-        # check that the output dim is identical
-        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"""
         prep = [qml.BasisState(np.array([1, 0]), wires=(0, 1))]
@@ -644,12 +590,6 @@ def test_deep_copy(self):
         assert all(m1 is not m2 for m1, m2 in zip(copied_qs.measurements, qs.measurements))
         assert copied_qs.shots is qs.shots
 
-        # check that the output dim is identical
-        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
 
@@ -763,10 +703,6 @@ def test_cached_properties_when_updating_operations(self):
         tape = QuantumScript(ops, measurements=[qml.counts()], shots=2500, trainable_params=[1])
 
         assert tape.batch_size == 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)]
@@ -776,10 +712,6 @@ def test_cached_properties_when_updating_operations(self):
         assert new_tape.operations == new_ops
 
         assert new_tape.batch_size == 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):
@@ -797,10 +729,6 @@ def test_cached_properties_when_updating_measurements(self):
 
         assert tape.obs_sharing_wires == []
         assert tape.obs_sharing_wires_id == []
-        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))]
@@ -809,10 +737,6 @@ def test_cached_properties_when_updating_measurements(self):
         assert tape.measurements == measurements
         assert new_tape.measurements == new_measurements
 
-        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]
@@ -1734,14 +1658,3 @@ 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