QiskitDevice.probabilities() changed to QiskitDevice.probability() to… (

#80)

* QiskitDevice.probabilities() changed to QiskitDevice.probability() to restore compatibility with pennylane python package

* updated changelog

Co-authored-by: antalszava <[email protected]>
Co-authored-by: Josh Izaac <[email protected]>

CHANGELOG.md

@@ -13,10 +13,15 @@
 
 ### Bug fixes
 
+* Renamed `QiskitDevice.probabilities` to `QiskitDevice.probability` to overload `pennylane.Device.probability`. This fixes a bug that raises `NotImplementedError` when a QNode is used to compute probabilities on a IBMQ device.
+ [(#80)](https://github.com/XanaduAI/pennylane-qiskit/pull/80)
+
 ### Contributors
 
 This release contains contributions from (in alphabetical order):
 
+Rafael Haenel
+
 ---
 
 # Release 0.8.2

pennylane_qiskit/qiskit_device.py

@@ -340,7 +340,7 @@ def expval(self, observable, wires, par):
         if self.backend_name in self._state_backends and self.analytic:
             # exact expectation value
             eigvals = self.eigvals(observable, wires, par)
-            prob = np.fromiter(self.probabilities(wires=wires).values(), dtype=np.float64)
+            prob = np.fromiter(self.probability(wires=wires).values(), dtype=np.float64)
             return (eigvals @ prob).real
 
         if self.analytic:
@@ -354,7 +354,7 @@ def var(self, observable, wires, par):
         if self.backend_name in self._state_backends and self.analytic:
             # exact variance value
             eigvals = self.eigvals(observable, wires, par)
-            prob = np.fromiter(self.probabilities(wires=wires).values(), dtype=np.float64)
+            prob = np.fromiter(self.probability(wires=wires).values(), dtype=np.float64)
             return (eigvals ** 2) @ prob - (eigvals @ prob).real ** 2
 
         if self.analytic:
@@ -371,7 +371,7 @@ def sample(self, observable, wires, par):
         if self.backend_name in self._state_backends:
             # software simulator. Need to sample from probabilities.
             eigvals = self.eigvals(observable, wires, par)
-            prob = np.fromiter(self.probabilities(wires=wires).values(), dtype=np.float64)
+            prob = np.fromiter(self.probability(wires=wires).values(), dtype=np.float64)
             return np.random.choice(eigvals, self.shots, p=prob)
 
         # a hardware simulator
@@ -385,7 +385,7 @@ def sample(self, observable, wires, par):
         else:
             # Need to convert counts into samples
             samples = np.vstack(
-                [np.vstack([s] * int(self.shots * p)) for s, p in self.probabilities().items()]
+                [np.vstack([s] * int(self.shots * p)) for s, p in self.probability().items()]
             )
 
         if isinstance(observable, str) and observable in {"PauliX", "PauliY", "PauliZ", "Hadamard"}:
@@ -405,7 +405,7 @@ def sample(self, observable, wires, par):
     def state(self):
         return self._state
 
-    def probabilities(self, wires=None):
+    def probability(self, wires=None):
         """Return the (marginal) probability of each computational basis
         state from the last run of the device.
 

tests/test_apply.py

@@ -200,7 +200,7 @@ def test_qubit_state_vector(self, init_state, device, tol):
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(state) ** 2
         assert np.allclose(res, expected, **tol)
 
@@ -224,7 +224,7 @@ def test_single_qubit_no_parameters(self, init_state, device, name, mat, tol):
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(mat @ state) ** 2
         assert np.allclose(res, expected, **tol)
 
@@ -240,7 +240,7 @@ def test_single_qubit_parameters(self, init_state, device, name, func, theta, to
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(func(theta) @ state) ** 2
         assert np.allclose(res, expected, **tol)
 
@@ -255,7 +255,7 @@ def test_two_qubit_no_parameters(self, init_state, device, name, mat, tol):
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(mat @ state) ** 2
         assert np.allclose(res, expected, **tol)
 
@@ -270,7 +270,7 @@ def test_qubit_unitary(self, init_state, device, mat, tol):
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(mat @ state) ** 2
         assert np.allclose(res, expected, **tol)
 
@@ -293,7 +293,7 @@ def test_three_qubit_no_parameters(self, init_state, device, name, mat, tol):
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(mat @ state) ** 2
         assert np.allclose(res, expected, **tol)
 
@@ -309,6 +309,6 @@ def test_single_qubit_parameters(self, init_state, device, name, func, theta, to
         dev._obs_queue = []
         dev.pre_measure()
 
-        res = np.fromiter(dev.probabilities().values(), dtype=np.float64)
+        res = np.fromiter(dev.probability().values(), dtype=np.float64)
         expected = np.abs(func(theta) @ state) ** 2
         assert np.allclose(res, expected, **tol)

tests/test_qiskit_device.py

@@ -35,7 +35,7 @@ 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
+        assert dev.probability() is None
 
 
 class TestAnalyticWarningHWSimulator: