Update LightningBaseFallBack to raise an error instead of falling back to default.qubit

.github/CHANGELOG.md

@@ -41,6 +41,9 @@
 
 ### Breaking changes
 
+* Update `lightning.gpu` and `lightning.kokkos` to raise an error instead of falling back to `default.qubit`.
+  [(#689)](https://github.com/PennyLaneAI/pennylane-lightning/pull/689)
+
 * Add `paths` directives to test workflows to avoid running tests that cannot be impacted by changes.
   [(#699)](https://github.com/PennyLaneAI/pennylane-lightning/pull/699)
   [(#695)](https://github.com/PennyLaneAI/pennylane-lightning/pull/695)

.github/workflows/tests_linux_python.yml

@@ -364,7 +364,7 @@ jobs:
           mv .coverage .coverage-${{ github.job }}-${{ matrix.pl_backend }}
           # TODO: Remove this if-cond with release v0.36.0
           if [ -f tests/test_native_mcm.py ]; then
-              OMP_NUM_THREADS=1 PL_DEVICE=${DEVICENAME} python -m pytest -n auto tests/ -k "test_native_mcm" $COVERAGE_FLAGS --cov-append
+              OMP_NUM_THREADS=1 PL_DEVICE=${DEVICENAME} python -m pytest -n auto tests/test_native_mcm.py $COVERAGE_FLAGS --cov-append
           fi
 
       - name: Install all backend devices
@@ -418,7 +418,7 @@ jobs:
       - name: Combine coverage files
         run: |
           python -m pip install coverage
-          python -m coverage combine .coverage-python*
+          python -m coverage combine .coverage*
           # Added cov xml -i to ignore "No source for code" random errors
           # https://stackoverflow.com/questions/2386975/no-source-for-code-message-in-coverage-py
           python -m coverage xml -i -o coverage-${{ github.job }}.xml

.github/workflows/wheel_linux_aarch64.yml

@@ -141,7 +141,7 @@ jobs:
 
           CIBW_BEFORE_TEST: |
             python -m pip install -r requirements-tests.txt
-            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" pip install -e . -vv; fi
+            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" python -m pip install . -vv; fi
 
           CIBW_TEST_COMMAND: |
             DEVICENAME=`echo ${{ matrix.pl_backend }} | sed "s/_/./g"`

.github/workflows/wheel_linux_x86_64.yml

@@ -156,7 +156,7 @@ jobs:
 
           CIBW_BEFORE_TEST: |
             python -m pip install -r requirements-tests.txt
-            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" pip install -e . -vv; fi
+            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" python -m pip install . -vv; fi
 
           CIBW_TEST_COMMAND: |
             DEVICENAME=`echo ${{ matrix.pl_backend }} | sed "s/_/./g"`

.github/workflows/wheel_macos_arm64.yml

@@ -96,7 +96,7 @@ jobs:
 
           CIBW_BEFORE_TEST: |
             python -m pip install -r requirements-tests.txt
-            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" pip install -e . -vv; fi
+            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" python -m pip install . -vv; fi
 
           CIBW_TEST_COMMAND: |
             DEVICENAME=`echo ${{ matrix.pl_backend }} | sed "s/_/./g"`

.github/workflows/wheel_macos_x86_64.yml

@@ -144,7 +144,7 @@ jobs:
 
           CIBW_BEFORE_TEST: |
             python -m pip install -r requirements-tests.txt
-            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" pip install -e . -vv; fi
+            if ${{ matrix.pl_backend == 'lightning_kokkos'}}; then SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" python -m pip install . -vv; fi
 
           CIBW_TEST_COMMAND: |
             DEVICENAME=`echo ${{ matrix.pl_backend }} | sed "s/_/./g"`

.github/workflows/wheel_win_x86_64.yml

@@ -173,7 +173,7 @@ jobs:
           python -m pip install setuptools
           python -m pip install -r requirements-tests.txt
           if (${{ matrix.pl_backend == 'lightning_kokkos'}}) {
-            SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" python -m pip install -e . -vv
+            SKIP_COMPILATION=True PL_BACKEND="lightning_qubit" python -m pip install . -vv
           }
           pushd wheelhouse
           $wheels = Get-ChildItem "./" -Filter *.whl

mpitests/test_adjoint_jacobian.py

@@ -28,6 +28,9 @@
 from pennylane import qnode
 from scipy.stats import unitary_group
 
+if not ld._CPP_BINARY_AVAILABLE:
+    pytest.skip("No binary module found. Skipping.", allow_module_level=True)
+
 I, X, Y, Z = (
     np.eye(2),
     qml.PauliX.compute_matrix(),
@@ -109,12 +112,10 @@ def test_not_expval(self, dev):
             qml.RX(0.1, wires=0)
             qml.state()
 
-        if device_name == "lightning.gpu" and ld._CPP_BINARY_AVAILABLE:
+        if device_name == "lightning.gpu":
             message = "Adjoint differentiation does not support State measurements."
-        elif ld._CPP_BINARY_AVAILABLE:
-            message = "This method does not support statevector return type."
         else:
-            message = "Adjoint differentiation method does not support measurement StateMP"
+            message = "This method does not support statevector return type."
         with pytest.raises(
             qml.QuantumFunctionError,
             match=message,
@@ -144,7 +145,6 @@ def test_empty_measurements(self, dev):
         jac = dev.adjoint_jacobian(tape)
         assert len(jac) == 0
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_unsupported_op(self, dev):
         """Test if a QuantumFunctionError is raised for an unsupported operation, i.e.,
         multi-parameter operations that are not qml.Rot"""
@@ -159,7 +159,6 @@ def test_unsupported_op(self, dev):
         ):
             dev.adjoint_jacobian(tape)
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_proj_unsupported(self, dev):
         """Test if a QuantumFunctionError is raised for a Projector observable"""
         with qml.tape.QuantumTape() as tape:
@@ -332,7 +331,6 @@ def test_multiple_rx_gradient_expval_hermitian(self, tol, dev):
     qubit_ops = [getattr(qml, name) for name in qml.ops._qubit__ops__]  # pylint: disable=no-member
     ops = {qml.RX, qml.RY, qml.RZ, qml.PhaseShift, qml.CRX, qml.CRY, qml.CRZ, qml.Rot}
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_multiple_rx_gradient_expval_hamiltonian(self, tol, dev):
         """Tests that the gradient of multiple RX gates in a circuit yields the correct result
         with Hermitian observable
@@ -522,7 +520,6 @@ def test_gradient_gate_with_multiple_parameters_hermitian(self, dev):
         # the different methods agree
         assert np.allclose(grad_D, grad_F, atol=tol, rtol=0)
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_gradient_gate_with_multiple_parameters_hamiltonian(self, dev):
         """Tests that gates with multiple free parameters yield correct gradients."""
         x, y, z = [0.5, 0.3, -0.7]
@@ -601,7 +598,6 @@ def test_provide_starting_state(self, tol, dev):
             dM2 = dev.adjoint_jacobian(tape, starting_state=state_vector)
             assert np.allclose(dM1, dM2, atol=tol, rtol=0)
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_provide_wrong_starting_state(self, dev):
         """Tests raise an exception when provided starting state mismatches."""
         x, y, z = [0.5, 0.3, -0.7]
@@ -624,7 +620,6 @@ def test_provide_wrong_starting_state(self, dev):
         device_name == "lightning.gpu",
         reason="Adjoint differentiation does not support State measurements.",
     )
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_state_return_type(self, dev):
         """Tests raise an exception when the return type is State"""
         with qml.tape.QuantumTape() as tape:
@@ -675,7 +670,6 @@ def circ(x):
         ):
             qml.grad(circ)(0.1)
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_qnode(self, mocker, dev):
         """Test that specifying diff_method allows the adjoint method to be selected"""
         args = np.array([0.54, 0.1, 0.5], requires_grad=True)
@@ -766,7 +760,6 @@ def cost(p1, p2):
 
         assert np.allclose(grad_D[0], expected, atol=tol, rtol=0)
 
-    @pytest.mark.skipif(not ld._CPP_BINARY_AVAILABLE, reason="Lightning binary required")
     def test_gradient_repeated_gate_parameters(self, mocker, dev):
         """Tests that repeated use of a free parameter in a multi-parameter gate yields correct
         gradients."""

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.36.0-dev43"
+__version__ = "0.36.0-dev44"

pennylane_lightning/core/lightning_base.py

@@ -22,7 +22,6 @@
 import numpy as np
 import pennylane as qml
 from pennylane import BasisState, QubitDevice, StatePrep
-from pennylane.devices import DefaultQubitLegacy
 from pennylane.measurements import Expectation, MeasurementProcess, State
 from pennylane.operation import Operation, Tensor
 from pennylane.ops import Prod, Projector, SProd, Sum
@@ -71,6 +70,12 @@ def __init__(
         shots=None,
         batch_obs=False,
     ):
+        if not self._CPP_BINARY_AVAILABLE:
+            raise ImportError(
+                f"Pre-compiled binaries for {self.short_name} are not available. "
+                "To manually compile from source, follow the instructions at "
+                "https://pennylane-lightning.readthedocs.io/en/latest/installation.html."
+            )
         if c_dtype is np.complex64:
             r_dtype = np.float32
             self.use_csingle = True
@@ -446,31 +451,3 @@ def processing_fns(tapes):
             return vjps
 
         return processing_fns
-
-
-class LightningBaseFallBack(DefaultQubitLegacy):  # pragma: no cover
-    # pylint: disable=missing-class-docstring, too-few-public-methods
-    pennylane_requires = ">=0.34"
-    version = __version__
-    author = "Xanadu Inc."
-    _CPP_BINARY_AVAILABLE = False
-    _new_API = False
-
-    def __init__(self, wires, *, c_dtype=np.complex128, **kwargs):
-        if c_dtype is np.complex64:
-            r_dtype = np.float32
-        elif c_dtype is np.complex128:
-            r_dtype = np.float64
-        else:
-            raise TypeError(f"Unsupported complex type: {c_dtype}")
-        super().__init__(wires, r_dtype=r_dtype, c_dtype=c_dtype, **kwargs)
-
-    @property
-    def state_vector(self):
-        """Returns a handle to the statevector."""
-        return self._state
-
-    @property
-    def dtype(self):
-        """State vector complex data type."""
-        return self.C_DTYPE