Deprecate top level access to legacy device base classes

.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

@@ -83,6 +83,10 @@
 
 <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.
+
 * `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
 
+ import warnings # pylint: disable=redefined-outer-name, import-outside-toplevel
+
  if name == "QubitDevice":
+ warnings.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":
+ warnings.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":
+ warnings.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

@@ -111,12 +111,6 @@ 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")
@@ -142,12 +136,6 @@ 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)