Deprecate top level access to legacy device base classes (#6238)

**Context:**

`qml.Device`, `qml.QubitDevice`, and `qml.QutritDevice` all reflect the
legacy device interface, which is no longer the recommended way of
creating devices.

**Description of the Change:**

Deprecate top level access to the `Device`, `QubitDevice`, and
`QutritDevice`.

**Benefits:**

Further isolation of the legacy device interface.

**Possible Drawbacks:**

All deprecations propagate through the ecosystem and can cause issues.

**Related GitHub Issues:**

[sc-71519]

---------

Co-authored-by: Mudit Pandey <mudit.pandey@xanadu.ai>

.github/workflows/interface-unit-tests.yml

@@ -379,8 +379,7 @@ jobs:
       # catalyst requires the latest version of pennylane that is about to be released.
       # Installing catalyst after pennylane to make sure that the latest catalyst is used.
       install_catalyst_nightly: true
-      # using lightning master does not work for the tests with external libraries
-      install_pennylane_lightning_master: false
+      install_pennylane_lightning_master: true
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: external
       additional_pip_packages: pyzx matplotlib stim quimb mitiq pennylane-qiskit ply

doc/development/deprecations.rst

@@ -9,6 +9,13 @@ deprecations are listed below.
 Pending deprecations
 --------------------
 
+* ``Device``, ``QubitDevice``, and ``QutritDevice`` will no longer be imported top level in v0.40.  They instead
+  we be available as ``qml.devices.LegacyDevice``, ``qml.devices.QubitDevice``, and ``qml.devices.QutritDevice``
+  respectively.
+
+  - Deprecated top level access in v0.39
+  - Top level access removed in v0.40
+
 * `QNode.gradient_fn` is deprecated. Please use `QNode.diff_method` instead. `QNode.get_gradient_fn` can also be used to
   process the diff method.
 

doc/releases/changelog-dev.md

@@ -94,6 +94,11 @@
 
 <h3>Deprecations 👋</h3>
 
+* `Device`, `QubitDevice`, and `QutritDevice` will no longer be accessible via top-level import in v0.40.
+  They will still be accessible as `qml.devices.LegacyDevice`, `qml.devices.QubitDevice`, and `qml.devices.QutritDevice`
+  respectively.
+  [(#6238)](https://github.com/PennyLaneAI/pennylane/pull/6238/)
+
 * `QNode.gradient_fn` is deprecated. Please use `QNode.diff_method` and `QNode.get_gradient_fn` instead.
   [(#6244)](https://github.com/PennyLaneAI/pennylane/pull/6244)
 

pennylane/__init__.py

@@ -16,8 +16,6 @@
 PennyLane can be directly imported.
 """
 
-import numpy as _np
-
 
 from pennylane.boolean_fn import BooleanFn
 import pennylane.numpy
@@ -180,13 +178,30 @@ def __getattr__(name):
     if name == "plugin_devices":
         return pennylane.devices.device_constructor.plugin_devices
 
+    from warnings import warn  # pylint: disable=import-outside-toplevel
+
     if name == "QubitDevice":
+        warn(
+            "QubitDevice will no longer be accessible top level. Please access "
+            " the class as pennylane.devices.QubitDevice",
+            PennyLaneDeprecationWarning,
+        )
         return pennylane.devices._qubit_device.QubitDevice  # pylint:disable=protected-access
 
     if name == "QutritDevice":
+        warn(
+            "QutritDevice will no longer be accessible top level. Please access "
+            " the class as pennylane.devices.QutritDevice",
+            PennyLaneDeprecationWarning,
+        )
         return pennylane.devices._qutrit_device.QutritDevice  # pylint:disable=protected-access
 
     if name == "Device":
+        warn(
+            "Device will no longer be accessible top level. Please access "
+            " the class as pennylane.devices.LegacyDevice",
+            PennyLaneDeprecationWarning,
+        )
         return pennylane.devices._legacy_device.Device  # pylint:disable=protected-access
 
     raise AttributeError(f"module 'pennylane' has no attribute '{name}'")

pennylane/devices/tests/conftest.py

@@ -110,12 +110,6 @@ def validate_diff_method(device, diff_method, device_kwargs):
     if diff_method == "backprop" and device_kwargs.get("shots") is not None:
         pytest.skip(reason="test should only be run in analytic mode")
     dev = device(1)
-    if isinstance(dev, qml.Device):
-        passthru_devices = dev.capabilities().get("passthru_devices")
-        if diff_method == "backprop" and passthru_devices is None:
-            pytest.skip(reason="device does not support backprop")
-        return
-
     config = qml.devices.ExecutionConfig(gradient_method=diff_method)
     if not dev.supports_derivatives(execution_config=config):
         pytest.skip(reason="device does not support diff_method")
@@ -141,12 +135,6 @@ def _device(wires):
                 f"plugin and all of its dependencies must be installed."
             )
 
-        if isinstance(dev, qml.Device):
-            capabilities = dev.capabilities()
-            if capabilities.get("model", None) != "qubit":
-                # exit the tests if device based on cv model (currently not supported)
-                pytest.exit("The device test suite currently only runs on qubit-based devices.")
-
         return dev
 
     return _device

pennylane/devices/tests/test_compare_default_qubit.py

@@ -38,9 +38,6 @@ def test_hermitian_expectation(self, device, tol, benchmark):
         if dev.shots:
             pytest.skip("Device is in non-analytical mode.")
 
-        if isinstance(dev, qml.Device) and "Hermitian" not in dev.observables:
-            pytest.skip("Device does not support the Hermitian observable.")
-
         if dev.name == "default.qubit":
             pytest.skip("Device is default.qubit.")
 
@@ -107,7 +104,7 @@ def test_projector_expectation(self, device, state, tol, benchmark):
         if dev.shots:
             pytest.skip("Device is in non-analytical mode.")
 
-        if isinstance(dev, qml.Device) and "Projector" not in dev.observables:
+        if isinstance(dev, qml.devices.LegacyDevice) and "Projector" not in dev.observables:
             pytest.skip("Device does not support the Projector observable.")
 
         if dev.name == "default.qubit":

pennylane/devices/tests/test_gates.py

@@ -358,7 +358,7 @@ def test_supported_gates_can_be_implemented(self, device_kwargs, operation):
         device_kwargs["wires"] = 4  # maximum size of current gates
         dev = qml.device(**device_kwargs)
 
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             if operation not in dev.operations:
                 pytest.skip("operation not supported.")
         else:
@@ -395,7 +395,7 @@ def test_basis_state(self, device, basis_state, tol, skip_if):
         """Test basis state initialization."""
         n_wires = 4
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         @qml.qnode(dev)
@@ -413,7 +413,7 @@ def test_state_prep(self, device, init_state, tol, skip_if):
         """Test StatePrep initialisation."""
         n_wires = 1
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         rnd_state = init_state(n_wires)
@@ -433,7 +433,7 @@ def test_single_qubit_no_parameters(self, device, init_state, op, mat, tol, skip
         """Test PauliX application."""
         n_wires = 1
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         rnd_state = init_state(n_wires)
@@ -457,7 +457,7 @@ def test_single_qubit_parameters(
         """Test single qubit gates taking a single scalar argument."""
         n_wires = 1
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         rnd_state = init_state(n_wires)
@@ -477,7 +477,7 @@ def test_rotation(self, device, init_state, tol, skip_if, benchmark):
         """Test three axis rotation gate."""
         n_wires = 1
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         rnd_state = init_state(n_wires)
@@ -501,7 +501,7 @@ def test_two_qubit_no_parameters(self, device, init_state, op, mat, tol, skip_if
         """Test two qubit gates."""
         n_wires = 2
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
             if not dev.supports_operation(op(wires=range(n_wires)).name):
                 pytest.skip("op not supported")
@@ -527,7 +527,7 @@ def test_two_qubit_parameters(
         """Test parametrized two qubit gates taking a single scalar argument."""
         n_wires = 2
         dev = device(n_wires)
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         rnd_state = init_state(n_wires)
@@ -549,7 +549,7 @@ def test_qubit_unitary(self, device, init_state, mat, tol, skip_if, benchmark):
         n_wires = int(np.log2(len(mat)))
         dev = device(n_wires)
 
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             if "QubitUnitary" not in dev.operations:
                 pytest.skip("Skipped because device does not support QubitUnitary.")
 
@@ -574,7 +574,7 @@ def test_special_unitary(self, device, init_state, theta_, tol, skip_if, benchma
         n_wires = int(np.log(len(theta_) + 1) / np.log(4))
         dev = device(n_wires)
 
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             if "SpecialUnitary" not in dev.operations:
                 pytest.skip("Skipped because device does not support SpecialUnitary.")
 
@@ -603,7 +603,7 @@ def test_three_qubit_no_parameters(self, device, init_state, op, mat, tol, skip_
         n_wires = 3
         dev = device(n_wires)
 
-        if isinstance(dev, qml.Device):
+        if isinstance(dev, qml.devices.LegacyDevice):
             skip_if(dev, {"returns_probs": False})
 
         rnd_state = init_state(n_wires)

pennylane/devices/tests/test_gates_with_expval.py

@@ -256,7 +256,7 @@ def test_supported_gate_two_wires_no_parameters(self, device, tol, name, expecte
         dev = device(n_wires)
 
         op = getattr(qml.ops, name)
-        if isinstance(dev, qml.Device) and not dev.supports_operation(op):
+        if isinstance(dev, qml.devices.LegacyDevice) and not dev.supports_operation(op):
             pytest.skip("operation not supported")
 
         @qml.qnode(dev)