Analytic true hw simulator warning (#59)

* Adding basic test cases and changing character setting (need polishing)

* Polish tests

* Add tests for the variance case

* Linting elif statement

* Correcting import order

* Organize warning message as a class attribute

* Reorganized tests of old non-native PL operations, added test for QubitUnitary transformation (ch gate)

* Revert "Reorganized tests of old non-native PL operations, added test for QubitUnitary transformation (ch gate)"

This reverts commit 7bb7641.

pennylane_qiskit/qiskit_device.py

@@ -29,26 +29,26 @@
 ~~~~~~~~~~~~
 """
 # pylint: disable=too-many-instance-attributes
+
 import abc
-from collections import OrderedDict
 import functools
 import inspect
 import itertools
+import warnings
+from collections import OrderedDict
 
 import numpy as np
-
+from qiskit import ClassicalRegister, QuantumCircuit, QuantumRegister
 from qiskit import extensions as ex
-from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
-
-from qiskit.compiler import transpile, assemble
 from qiskit.circuit.measure import measure
-from qiskit.converters import dag_to_circuit, circuit_to_dag
+from qiskit.compiler import assemble, transpile
+from qiskit.converters import circuit_to_dag, dag_to_circuit
 
 from pennylane import Device, DeviceError
 from pennylane.operation import Sample
 
-from .gates import BasisState, Rot
 from ._version import __version__
+from .gates import BasisState, Rot
 
 
 @functools.lru_cache()
@@ -134,6 +134,10 @@ class QiskitDevice(Device, abc.ABC):
     operations = set(_operation_map.keys())
     observables = {"PauliX", "PauliY", "PauliZ", "Identity", "Hadamard", "Hermitian"}
 
+    hw_analytic_warning_message = "The analytic calculation of expectations and variances "\
+                                  "is only supported on statevector backends, not on the {}. "\
+                                  "The obtained result is based on sampling."
+
     _eigs = {}
 
     def __init__(self, wires, provider, backend, shots=1024, **kwargs):
@@ -349,6 +353,12 @@ def expval(self, observable, wires, par):
             prob = np.fromiter(self.probabilities(wires=wires).values(), dtype=np.float64)
             return (eigvals @ prob).real
 
+        if self.analytic:
+            # Raise a warning if backend is a hardware simulator
+            warnings.warn(self.hw_analytic_warning_message.
+                          format(self.backend),
+                          UserWarning)
+
         # estimate the ev
         return np.mean(self.sample(observable, wires, par))
 
@@ -359,6 +369,12 @@ def var(self, observable, wires, par):
             prob = np.fromiter(self.probabilities(wires=wires).values(), dtype=np.float64)
             return (eigvals ** 2) @ prob - (eigvals @ prob).real ** 2
 
+        if self.analytic:
+            # Raise a warning if backend is a hardware simulator
+            warnings.warn(self.hw_analytic_warning_message.
+                          format(self.backend),
+                          UserWarning)
+
         return np.var(self.sample(observable, wires, par))
 
     def sample(self, observable, wires, par):

tests/conftest.py

@@ -48,6 +48,16 @@ def backend(request):
     return request.param
 
 
+@pytest.fixture(params=state_backends)
+def statevector_backend(request):
+    return request.param
+
+
+@pytest.fixture(params=hw_backends)
+def hardware_backend(request):
+    return request.param
+
+
 @pytest.fixture(params=[AerDevice, BasicAerDevice])
 def device(request, backend, shots, analytic):
     if backend not in state_backends and analytic == True:

tests/test_converter.py

@@ -691,8 +691,7 @@ class TestConverterWarnings:
     """Tests that the converter.load function emits warnings."""
 
     def test_barrier_not_supported(self, recorder):
-        """Tests the load method raises a ValueError, if something that is
-        not a QuanctumCircuit was passed."""
+        """Tests that a warning is raised if an unsupported instruction was reached."""
         qc = QuantumCircuit(3, 1)
         qc.barrier()
 

tests/test_qiskit_device.py

@@ -1,39 +1,116 @@
-import pytest
-
-import numpy as np
-
-from pennylane_qiskit.qiskit_device import pauli_eigs
-from pennylane_qiskit import AerDevice
-
-
-Z = np.diag([1, -1])
-
-
-class TestZEigs:
-    r"""Test that eigenvalues of Z^{\otimes n} are correctly generated"""
-
-    def test_one(self):
-        """Test that eigs(Z) = [1, -1]"""
-        assert np.all(pauli_eigs(1) == np.array([1, -1]))
-
-    @pytest.mark.parametrize("n", [2, 3, 6])
-    def test_multiple(self, n):
-        r"""Test that eigs(Z^{\otimes n}) is correct"""
-        res = pauli_eigs(n)
-        Zn = np.kron(Z, Z)
-
-        for _ in range(n - 2):
-            Zn = np.kron(Zn, Z)
-
-        expected = np.diag(Zn)
-        assert np.all(res == expected)
-
-
-class TestProbabilities:
-    """Tests for the probability function"""
-
-    def test_probability_no_results(self):
-        """Test that the probabilities function returns
-        None if no job has yet been run."""
-        dev = AerDevice(backend="statevector_simulator", wires=1, analytic=True)
-        assert dev.probabilities() is None
+import numpy as np
+import pytest
+
+import pennylane as qml
+from pennylane_qiskit import AerDevice
+from pennylane_qiskit.qiskit_device import pauli_eigs
+
+Z = np.diag([1, -1])
+
+
+class TestZEigs:
+    r"""Test that eigenvalues of Z^{\otimes n} are correctly generated"""
+
+    def test_one(self):
+        """Test that eigs(Z) = [1, -1]"""
+        assert np.all(pauli_eigs(1) == np.array([1, -1]))
+
+    @pytest.mark.parametrize("n", [2, 3, 6])
+    def test_multiple(self, n):
+        r"""Test that eigs(Z^{\otimes n}) is correct"""
+        res = pauli_eigs(n)
+        Zn = np.kron(Z, Z)
+
+        for _ in range(n - 2):
+            Zn = np.kron(Zn, Z)
+
+        expected = np.diag(Zn)
+        assert np.all(res == expected)
+
+
+class TestProbabilities:
+    """Tests for the probability function"""
+
+    def test_probability_no_results(self):
+        """Test that the probabilities function returns
+        None if no job has yet been run."""
+        dev = AerDevice(backend="statevector_simulator", wires=1, analytic=True)
+        assert dev.probabilities() is None
+
+
+class TestAnalyticWarningHWSimulator:
+    """Tests the warnings for when the analytic attribute of a device is set to true"""
+
+    def test_warning_raised_for_hardware_backend_analytic_expval(self, hardware_backend, recorder):
+        """Tests that a warning is raised if the analytic attribute is true on
+            hardware simulators when calculating the expectation"""
+
+        dev = qml.device("qiskit.basicaer", backend=hardware_backend, wires=2)
+
+        @qml.qnode(dev)
+        def circuit():
+            qml.Hadamard(wires=0)
+            return qml.expval(qml.PauliZ(0))
+
+        with pytest.warns(UserWarning) as record:
+            circuit()
+
+        # check that only one warning was raised
+        assert len(record) == 1
+        # check that the message matches
+        assert record[0].message.args[0] == "The analytic calculation of expectations and variances "\
+                                            "is only supported on statevector backends, not on the {}. "\
+                                            "The obtained result is based on sampling.".format(dev.backend)
+
+    def test_no_warning_raised_for_software_backend_analytic_expval(self, statevector_backend, recorder, recwarn):
+        """Tests that no warning is raised if the analytic attribute is true on
+            statevector simulators when calculating the expectation"""
+
+        dev = qml.device("qiskit.basicaer", backend=statevector_backend, wires=2)
+
+        @qml.qnode(dev)
+        def circuit():
+            qml.Hadamard(wires=0)
+            return qml.expval(qml.PauliZ(0))
+
+        circuit()
+
+        # check that no warnings were raised
+        assert len(recwarn) == 0
+
+    def test_warning_raised_for_hardware_backend_analytic_var(self, hardware_backend, recorder):
+        """Tests that a warning is raised if the analytic attribute is true on
+            hardware simulators when calculating the variance"""
+
+        dev = qml.device("qiskit.basicaer", backend=hardware_backend, wires=2)
+
+        @qml.qnode(dev)
+        def circuit():
+            qml.Hadamard(wires=0)
+            return qml.var(qml.PauliZ(0))
+
+        with pytest.warns(UserWarning) as record:
+            circuit()
+
+        # check that only one warning was raised
+        assert len(record) == 1
+        # check that the message matches
+        assert record[0].message.args[0] == "The analytic calculation of expectations and variances "\
+                                            "is only supported on statevector backends, not on the {}. "\
+                                            "The obtained result is based on sampling.".format(dev.backend)
+
+    def test_no_warning_raised_for_software_backend_analytic_var(self, statevector_backend, recorder, recwarn):
+        """Tests that no warning is raised if the analytic attribute is true on
+            statevector simulators when calculating the variance"""
+
+        dev = qml.device("qiskit.basicaer", backend=statevector_backend, wires=2)
+
+        @qml.qnode(dev)
+        def circuit():
+            qml.Hadamard(wires=0)
+            return qml.var(qml.PauliZ(0))
+
+        circuit()
+
+        # check that no warnings were raised
+        assert len(recwarn) == 0