Remove old opmath (#6548)

**Context:** It's time to fully remove the legacy operator arithmetic (`Tensor` and `qml.ops.Hamiltonian`, and all of the structure surrounding them). **Description of the Change:** - [x] remove `qml.ops.Hamiltonian` - [x] remove `Tensor` - [x] remove `disable_` and `enable_new_opmath` - [x] remove `operation.convert_to_legacy_H` - [x] remove `qml.pauli.simplify` - [x] remove `PauliWord.hamiltonian` and `PauliSentence.hamiltonian`, and any other functionality in the pauli module that supported legacy opmath - [x] remove any if/else handling in place to deal with legacy opmath through the code base - [x] remove `legacy_opmath_only`, `new_opmath_only` and `use_legacy_and_new_opmath` test fixtures, and all irrelevant tests - [x] remove the warning suppressions added in [PR6287](#6287) - [x] remove the legacy test fixture (--disable-opmath) [sc-77523] --------- Co-authored-by: Christina Lee <christina@xanadu.ai> Co-authored-by: Astral Cai <astral.cai@xanadu.ai>
PennyLaneAI · Nov 17, 2024 · 97c852b · 97c852b
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diff --git a/.github/workflows/interface-unit-tests.yml b/.github/workflows/interface-unit-tests.yml
@@ -54,11 +54,6 @@ on:
         required: false
         type: string
         default: ''
-      disable_new_opmath:
-        description: Whether to disable the new op_math or not when running the tests
-        required: false
-        type: string
-        default: "False"
       additional_python_packages:
         description: Additional Python packages to install separated by a space
         required: false
@@ -246,7 +241,7 @@ jobs:
         ${{ needs.default-dependency-versions.outputs.pytorch-version }}
         ${{ inputs.additional_python_packages }}
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: torch and not qcut and not finite-diff and not param-shift
       requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'torch.txt' || '' }}
@@ -282,7 +277,7 @@ jobs:
       additional_pip_packages: ${{ inputs.additional_python_packages }}
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       pytest_markers: autograd and not qcut and not finite-diff and not param-shift
 
 
@@ -320,7 +315,7 @@ jobs:
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: tf and not qcut and not finite-diff and not param-shift
-      pytest_additional_args: --splits 3 --group ${{ matrix.group }} --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: --splits 3 --group ${{ matrix.group }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       pytest_durations_file_path: '.github/durations/tf_tests_durations.json'
       requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'tf.txt' || '' }}
 
@@ -358,7 +353,7 @@ jobs:
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: jax and not qcut and not finite-diff and not param-shift
-      pytest_additional_args: --dist=loadscope --splits 4 --group ${{ matrix.group }} --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: --dist=loadscope --splits 4 --group ${{ matrix.group }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       pytest_durations_file_path: '.github/durations/jax_tests_durations.json'
       requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'jax.txt' || '' }}
 
@@ -395,7 +390,7 @@ jobs:
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: core and not qcut and not finite-diff and not param-shift
-      pytest_additional_args: --splits 6 --group ${{ matrix.group }} --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: --splits 6 --group ${{ matrix.group }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       pytest_durations_file_path: '.github/durations/core_tests_durations.json'
       requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'core.txt' || '' }}
 
@@ -435,7 +430,7 @@ jobs:
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: all_interfaces
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'all_interfaces.txt' || '' }}
 
 
@@ -468,7 +463,7 @@ jobs:
       python_version: ${{ matrix.python-version }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: external
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       additional_pip_packages: |
         pyzx matplotlib stim quimb mitiq ply
         git+https://github.com/PennyLaneAI/pennylane-qiskit.git@master
@@ -515,7 +510,7 @@ jobs:
       python_version: ${{ matrix.python-version }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: qcut
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       additional_pip_packages: |
         kahypar==1.1.7 
         opt_einsum
@@ -557,7 +552,7 @@ jobs:
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: qchem
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       additional_pip_packages: |
         openfermionpyscf basis-set-exchange
         ${{ inputs.additional_python_packages }}
@@ -592,7 +587,7 @@ jobs:
       branch: ${{ inputs.branch }}
       coverage_artifact_name: gradients-${{ matrix.config.suite }}-coverage
       python_version: ${{ matrix.python-version }}
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       additional_pip_packages: |
         ${{ needs.default-dependency-versions.outputs.jax-version }}
         ${{ needs.default-dependency-versions.outputs.tensorflow-version }}
@@ -632,7 +627,7 @@ jobs:
       python_version: ${{ matrix.python-version }}
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
-      pytest_additional_args: --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
       pytest_markers: data
       additional_pip_packages: |
         h5py
@@ -681,7 +676,7 @@ jobs:
       additional_pip_packages_post: ${{ needs.default-dependency-versions.outputs.pennylane-lightning-latest }}
       pytest_test_directory: pennylane/devices/tests
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
-      pytest_additional_args: --device=${{ matrix.config.device }} --shots=${{ matrix.config.shots }} --disable-opmath=${{ inputs.disable_new_opmath }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
+      pytest_additional_args: --device=${{ matrix.config.device }} --shots=${{ matrix.config.shots }} -W ${{ inputs.python_warning_level }} ${{ inputs.python_warning_level != 'default' && '--continue-on-collection-errors' || '' }}
 
 
   upload-to-codecov:

diff --git a/.github/workflows/legacy_op_math.yml b/.github/workflows/legacy_op_math.yml
diff --git a/doc/conf.py b/doc/conf.py
@@ -113,8 +113,8 @@
 # built documents.
 
 import pennylane
+pennylane.Hamiltonian = pennylane.ops.op_math.linear_combination.Hamiltonian
 
-pennylane.Hamiltonian = pennylane.ops.Hamiltonian
 
 # The full version, including alpha/beta/rc tags.
 release = pennylane.__version__

diff --git a/doc/development/deprecations.rst b/doc/development/deprecations.rst
@@ -44,58 +44,47 @@ Pending deprecations
   - Deprecated in v0.39
   - Will be removed in v0.40
 
-New operator arithmetic deprecations
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-In PennyLane v0.39, the legacy operator arithmetic system has been deprecated. Check out the :ref:`Updated operators <new_opmath>` page
-for details on how to port your legacy code to the new system. The old system is still accessible via :func:`~.disable_new_opmath`, though
-it is not recommended, as the old system is deprecated and will be removed in the v0.40 release. The following functionality will explicitly
-raise a deprecation warning when used:
-
-* In PennyLane v0.39, legacy operator arithmetic has been deprecated. This includes :func:`~pennylane.operation.enable_new_opmath`,
-  :func:`~pennylane.operation.disable_new_opmath`, :class:`~pennylane.ops.Hamiltonian`, and :class:`~pennylane.operation.Tensor`. Note
-  that when new operator arithmetic is enabled, ``qml.Hamiltonian`` will continue to dispatch to :class:`~pennylane.ops.LinearCombination`;
-  this behaviour is not deprecated.
-
-  - Deprecated in v0.39
-  - Will be removed in v0.40
-
-* :meth:`~pennylane.pauli.PauliSentence.hamiltonian` and :meth:`~pennylane.pauli.PauliWord.hamiltonian` are deprecated. Instead, please use
-  :meth:`~pennylane.pauli.PauliSentence.operation` and :meth:`~pennylane.pauli.PauliWord.operation` respectively.
-
-  - Deprecated in v0.39
-  - Will be removed in v0.40
-
-* :func:`pennylane.pauli.simplify` is deprecated. Instead, please use :func:`pennylane.simplify` or :meth:`~pennylane.operation.Operator.simplify`.
-
-  - Deprecated in v0.39
-  - Will be removed in v0.40
-
-* ``op.ops`` and ``op.coeffs`` will be deprecated in the future. Use 
+* ``op.ops`` and ``op.coeffs`` for ``Sum`` and ``Prod`` will be removed in the future. Use
   :meth:`~.Operator.terms` instead.
 
-  - Added and deprecated for ``Sum`` and ``Prod`` instances in v0.35
+  - deprecated in v0.35
 
 * Accessing terms of a tensor product (e.g., ``op = X(0) @ X(1)``) via ``op.obs`` is deprecated with new operator arithmetic.
   A user should use :class:`op.operands <~.CompositeOp>` instead.
 
   - Deprecated in v0.36
 
-
-Other deprecations
-~~~~~~~~~~~~~~~~~~
-
-* PennyLane Lightning and Catalyst will no longer support ``manylinux2014`` (GLIBC 2.17) compatibile Linux operating systems, and will be migrated to ``manylinux_2_28`` (GLIBC 2.28). See `pypa/manylinux <https://github.com/pypa/manylinux>`_ for additional details.
-
-  - Last supported version of ``manylinux2014`` with v0.36
-  - Fully migrated to ``manylinux_2_28`` with v0.37
-
 * ``MultiControlledX`` is the only controlled operation that still supports specifying control
   values with a bit string. In the future, it will no longer accepts strings as control values.
 
   - Deprecated in v0.36
   - Will be removed in v0.37
 
+Completed removal of legacy operator arithmetic
+-----------------------------------------------
+
+In PennyLane v0.40, the legacy operator arithmetic system has been removed, and is fully replaced by the new
+operator arithmetic functionality that was introduced in v0.36. Check out the :ref:`Updated operators <new_opmath>` page
+for details on how to port your legacy code to the new system. The following functionality has been removed:
+
+* In PennyLane v0.40, legacy operator arithmetic has been removed. This includes :func:`~pennylane.operation.enable_new_opmath`,
+  :func:`~pennylane.operation.disable_new_opmath`, :class:`~pennylane.ops.Hamiltonian`, and :class:`~pennylane.operation.Tensor`. Note
+  that ``qml.Hamiltonian`` will continue to dispatch to :class:`~pennylane.ops.LinearCombination`.
+
+  - Deprecated in v0.39
+  - Removed in v0.40
+
+* :meth:`~pennylane.pauli.PauliSentence.hamiltonian` and :meth:`~pennylane.pauli.PauliWord.hamiltonian` has been removed. Instead, please use
+  :meth:`~pennylane.pauli.PauliSentence.operation` and :meth:`~pennylane.pauli.PauliWord.operation` respectively.
+
+  - Deprecated in v0.39
+  - Removed in v0.40
+
+* :func:`pennylane.pauli.simplify` has been removed. Instead, please use :func:`pennylane.simplify` or :meth:`~pennylane.operation.Operator.simplify`.
+
+  - Deprecated in v0.39
+  - Removed in v0.40
+
 Completed deprecation cycles
 ----------------------------
 
@@ -164,6 +153,11 @@ Completed deprecation cycles
   - Deprecated in v0.39
   - Removed in v0.40
 
+* PennyLane Lightning and Catalyst will no longer support ``manylinux2014`` (GLIBC 2.17) compatibile Linux operating systems, and will be migrated to ``manylinux_2_28`` (GLIBC 2.28). See `pypa/manylinux <https://github.com/pypa/manylinux>`_ for additional details.
+
+  - Last supported version of ``manylinux2014`` with v0.36
+  - Fully migrated to ``manylinux_2_28`` with v0.37
+
 * The ``simplify`` argument in ``qml.Hamiltonian`` and ``qml.ops.LinearCombination`` has been removed.
   Instead, ``qml.simplify()`` can be called on the constructed operator.
 

diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
@@ -66,6 +66,12 @@
 
 <h3>Breaking changes 💔</h3>
 
+* Legacy operator arithmetic has been removed. This includes `qml.ops.Hamiltonian`, `qml.operation.Tensor`,
+  `qml.operation.enable_new_opmath`, `qml.operation.disable_new_opmath`, and `qml.operation.convert_to_legacy_H`.
+  Note that `qml.Hamiltonian` will continue to dispatch to `qml.ops.LinearCombination`. For more information, 
+  check out the [updated operator troubleshooting page](https://docs.pennylane.ai/en/stable/news/new_opmath.html).
+  [(#6548)](https://github.com/PennyLaneAI/pennylane/pull/6548)
+
 * The developer-facing `qml.utils` module has been removed. Specifically, the
 following 4 sets of functions have been either moved or removed[(#6588)](https://github.com/PennyLaneAI/pennylane/pull/6588):
 

diff --git a/pennylane/__init__.py b/pennylane/__init__.py
@@ -74,6 +74,7 @@
 )
 from pennylane.ops import *
 from pennylane.ops import adjoint, ctrl, cond, exp, sum, pow, prod, s_prod
+from pennylane.ops import LinearCombination as Hamiltonian
 from pennylane.templates import layer
 from pennylane.templates.embeddings import *
 from pennylane.templates.layers import *
@@ -165,14 +166,7 @@ class PennyLaneDeprecationWarning(UserWarning):
     """Warning raised when a PennyLane feature is being deprecated."""
 
 
-del globals()["Hamiltonian"]
-
-
 def __getattr__(name):
-    if name == "Hamiltonian":
-        if pennylane.operation.active_new_opmath():
-            return pennylane.ops.LinearCombination
-        return pennylane.ops.Hamiltonian
 
     if name == "plugin_devices":
         return pennylane.devices.device_constructor.plugin_devices

diff --git a/pennylane/data/attributes/operator/operator.py b/pennylane/data/attributes/operator/operator.py
@@ -24,7 +24,7 @@
 import pennylane as qml
 from pennylane.data.base.attribute import DatasetAttribute
 from pennylane.data.base.hdf5 import HDF5Group, h5py
-from pennylane.operation import Operator, Tensor
+from pennylane.operation import Operator
 
 from ._wires import wires_to_json
 
@@ -44,8 +44,6 @@ class DatasetOperator(Generic[Op], DatasetAttribute[HDF5Group, Op, Op]):
             arguments.
         - Hyperparameters are not used or are automatically derived by ``__init__()``.
 
-    Almost all operators meet these conditions. This type also supports serializing the
-    ``Hamiltonian`` and ``Tensor`` operators.
     """
 
     type_id = "operator"
@@ -56,13 +54,9 @@ def supported_ops(cls) -> frozenset[Type[Operator]]:
         """Set of supported operators."""
         return frozenset(
             (
-                # pennylane/operation/Tensor
-                Tensor,
                 # pennylane/ops/qubit/arithmetic_qml.py
                 qml.QubitCarry,
                 qml.QubitSum,
-                # pennylane/ops/qubit/hamiltonian.py
-                qml.ops.Hamiltonian,
                 # pennylane/ops/op_math/linear_combination.py
                 qml.ops.LinearCombination,
                 # pennylane/ops/op_math - prod.py, s_prod.py, sum.py
@@ -219,10 +213,7 @@ def _ops_to_hdf5(
                     f"Serialization of operator type '{type(op).__name__}' is not supported."
                 )
 
-            if isinstance(op, Tensor):
-                self._ops_to_hdf5(bind, op_key, op.obs)
-                op_wire_labels.append("null")
-            elif isinstance(op, (qml.ops.Hamiltonian, qml.ops.LinearCombination)):
+            if isinstance(op, qml.ops.LinearCombination):
                 coeffs, ops = op.terms()
                 ham_grp = self._ops_to_hdf5(bind, op_key, ops)
                 ham_grp["hamiltonian_coeffs"] = coeffs
@@ -260,10 +251,7 @@ def _hdf5_to_ops(self, bind: HDF5Group) -> list[Operator]:
                 op_key = f"op_{i}"
 
                 op_cls = self._supported_ops_dict()[op_class_name]
-                if op_cls is Tensor:
-                    prod_op = qml.ops.Prod if qml.operation.active_new_opmath() else Tensor
-                    ops.append(prod_op(*self._hdf5_to_ops(bind[op_key])))
-                elif op_cls in (qml.ops.Hamiltonian, qml.ops.LinearCombination):
+                if op_cls is qml.ops.LinearCombination:
                     ops.append(
                         qml.Hamiltonian(
                             coeffs=list(bind[op_key]["hamiltonian_coeffs"]),