Remove decomp_depth from qml.device constructor

doc/development/deprecations.rst

@@ -21,11 +21,6 @@ Pending deprecations
   - Deprecated in v0.38
   - Will be removed in v0.39  
 
-* The ``decomp_depth`` argument in ``qml.device`` is deprecated. 
-
-  - Deprecated in v0.38
-  - Will be removed in v0.39
-
 * The functions ``qml.qinfo.classical_fisher`` and ``qml.qinfo.quantum_fisher`` are deprecated since they are being migrated
   to the ``qml.gradients`` module. Therefore, ``qml.gradients.classical_fisher`` and ``qml.gradients.quantum_fisher`` should be used instead.
 
@@ -82,6 +77,11 @@ Other deprecations
 Completed deprecation cycles
 ----------------------------
 
+* The ``decomp_depth`` argument in ``qml.device`` is removed. 
+
+  - Deprecated in v0.38
+  - Removed in v0.39
+
 * The ``expansion_strategy`` attribute of ``qml.QNode`` is removed.
   Users should make use of ``qml.workflow.construct_batch``, should they require fine control over the output tape(s).
 

doc/releases/changelog-dev.md

@@ -39,6 +39,8 @@
   Please use `qml.transforms.split_non_commuting` instead.
   [(#6204)](https://github.com/PennyLaneAI/pennylane/pull/6204)
 
+* The `decomp_depth` keyword argument to `qml.device` is removed.
+
 <h3>Deprecations 👋</h3>
 
 <h3>Documentation 📝</h3>

pennylane/devices/device_constructor.py

@@ -14,7 +14,6 @@
 """
 This module contains code for the main device construction delegation logic.
 """
-import warnings
 from importlib import metadata
 from sys import version_info
 
@@ -57,37 +56,36 @@ def refresh_devices():
 
 # pylint: disable=protected-access
 def device(name, *args, **kwargs):
-    r"""
-    Load a device and return the instance.
+    r"""Load a device and return the instance.
 
     This function is used to load a particular quantum device,
     which can then be used to construct QNodes.
 
     PennyLane comes with support for the following devices:
 
     * :mod:`'default.qubit' <pennylane.devices.default_qubit>`: a simple
-      state simulator of qubit-based quantum circuit architectures.
+        state simulator of qubit-based quantum circuit architectures.
 
     * :mod:`'default.mixed' <pennylane.devices.default_mixed>`: a mixed-state
-      simulator of qubit-based quantum circuit architectures.
+        simulator of qubit-based quantum circuit architectures.
 
     * ``'lightning.qubit'``: a more performant state simulator of qubit-based
-      quantum circuit architectures written in C++.
+        quantum circuit architectures written in C++.
 
     * :mod:`'default.qutrit' <pennylane.devices.default_qutrit>`: a simple
-      state simulator of qutrit-based quantum circuit architectures.
+        state simulator of qutrit-based quantum circuit architectures.
 
     * :mod:`'default.qutrit.mixed' <pennylane.devices.default_qutrit_mixed>`: a
-      mixed-state simulator of qutrit-based quantum circuit architectures.
+        mixed-state simulator of qutrit-based quantum circuit architectures.
 
     * :mod:`'default.gaussian' <pennylane.devices.default_gaussian>`: a simple simulator
-      of Gaussian states and operations on continuous-variable circuit architectures.
+        of Gaussian states and operations on continuous-variable circuit architectures.
 
     * :mod:`'default.clifford' <pennylane.devices.default_clifford>`: an efficient
-      simulator of Clifford circuits.
+        simulator of Clifford circuits.
 
     * :mod:`'default.tensor' <pennylane.devices.default_tensor>`: a simulator
-      of quantum circuits based on tensor networks.
+        of quantum circuits based on tensor networks.
 
     Additional devices are supported through plugins — see
     the  `available plugins <https://pennylane.ai/plugins.html>`_ for more
@@ -105,12 +103,6 @@ def device(name, *args, **kwargs):
             that contains global and/or device specific configurations.
         custom_decomps (Dict[Union(str, Operator), Callable]): Custom
             decompositions to be applied by the device at runtime.
-        decomp_depth (int): For when custom decompositions are specified,
-            the maximum expansion depth used by the expansion function.
-
-    .. warning::
-
-        The ``decomp_depth`` argument is deprecated and will be removed in version 0.39.
 
     All devices must be loaded by specifying their **short-name** as listed above,
     followed by the **wires** (subsystems) you wish to initialize. The ``wires``
@@ -122,10 +114,10 @@ def device(name, *args, **kwargs):
         dev = qml.device('default.qubit', wires=5)
 
         def circuit():
-           qml.Hadamard(wires=1)
-           qml.Hadamard(wires=[0])
-           qml.CNOT(wires=[3, 4])
-           ...
+            qml.Hadamard(wires=1)
+            qml.Hadamard(wires=[0])
+            qml.CNOT(wires=[3, 4])
+            ...
 
     The ``wires`` argument can also be a sequence of unique numbers or strings, specifying custom wire labels
     that the user employs to address the wires:
@@ -135,10 +127,10 @@ def circuit():
         dev = qml.device('default.qubit', wires=['ancilla', 'q11', 'q12', -1, 1])
 
         def circuit():
-           qml.Hadamard(wires='q11')
-           qml.Hadamard(wires=['ancilla'])
-           qml.CNOT(wires=['q12', -1])
-           ...
+            qml.Hadamard(wires='q11')
+            qml.Hadamard(wires=['ancilla'])
+            qml.CNOT(wires=['q12', -1])
+            ...
 
     On some newer devices, such as ``default.qubit``, the ``wires`` argument can be omitted altogether,
     and instead the wires will be computed when executing a circuit depending on its contents.
@@ -157,8 +149,8 @@ def circuit():
 
         @qml.qnode(dev)
         def circuit(a):
-          qml.RX(a, wires=0)
-          return qml.sample(qml.Z(0))
+            qml.RX(a, wires=0)
+            return qml.sample(qml.Z(0))
 
     >>> circuit(0.8)  # 10 samples are returned
     array([ 1,  1,  1,  1, -1,  1,  1, -1,  1,  1])
@@ -243,15 +235,6 @@ def run_cnot():
         # Pop the custom decomposition keyword argument; we will use it here
         # only and not pass it to the device.
         custom_decomps = kwargs.pop("custom_decomps", None)
-        decomp_depth = kwargs.pop("decomp_depth", None)
-
-        if decomp_depth is not None:
-            warnings.warn(
-                "The decomp_depth argument is deprecated and will be removed in version 0.39. ",
-                qml.PennyLaneDeprecationWarning,
-            )
-        else:
-            decomp_depth = 10
 
         kwargs.pop("config", None)
         options.update(kwargs)
@@ -284,12 +267,12 @@ def _safe_specifier_set(version_str):
         if custom_decomps is not None:
             if isinstance(dev, qml.devices.LegacyDevice):
                 custom_decomp_expand_fn = qml.transforms.create_decomp_expand_fn(
-                    custom_decomps, dev, decomp_depth=decomp_depth
+                    custom_decomps, dev, decomp_depth=10
                 )
                 dev.custom_expand(custom_decomp_expand_fn)
             else:
                 custom_decomp_preprocess = qml.transforms.tape_expand._create_decomp_preprocessing(
-                    custom_decomps, dev, decomp_depth=decomp_depth
+                    custom_decomps, dev, decomp_depth=10
                 )
                 dev.preprocess = custom_decomp_preprocess