From 35d8d18b90b8fc347f18ad93c58d3f93e5621e3c Mon Sep 17 00:00:00 2001
From: antalszava <antalszava@gmail.com>
Date: Tue, 9 Feb 2021 13:56:27 -0500
Subject: [PATCH] No backprop devices (#61)

* Getting lighting

* revert test hack

* formatting

* Revert "formatting"

This reverts commit b2a082c2b4dd3da91cbbd35d74ec4d9351e14572.

* formatting

* Revert "revert test hack"

This reverts commit e410e3b045b04b50968a890593ca85965e1758f1.

* test no backprop

* qfunc

* Update tests/test_apply.py

Co-authored-by: Josh Izaac <josh146@gmail.com>

* test best diff method

* changelog

Co-authored-by: Josh Izaac <josh146@gmail.com>
---
 CHANGELOG.md                           |  7 +++++++
 pennylane_lightning/lightning_qubit.py |  1 +
 tests/test_apply.py                    | 23 +++++++++++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 24ff2ca09..e6541a75c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,10 +10,17 @@
 
 ### Bug fixes
 
+* Fixes a bug where the `QNode` would swap `LightningQubit` to
+  `DefaultQubitAutograd` on device execution due to the inherited
+  `passthru_devices` entry of the `capabilities` dictionary.
+  [(#61)](https://github.com/PennyLaneAI/pennylane-lightning/pull/61)
+
 ### Contributors
 
 This release contains contributions from (in alphabetical order):
 
+Josh Izaac, Antal Száva
+
 ---
 
 # Release 0.14.0
diff --git a/pennylane_lightning/lightning_qubit.py b/pennylane_lightning/lightning_qubit.py
index 52476aad9..b2c6b9d5f 100644
--- a/pennylane_lightning/lightning_qubit.py
+++ b/pennylane_lightning/lightning_qubit.py
@@ -107,6 +107,7 @@ def capabilities(cls):
             supports_analytic_computation=True,
             returns_state=True,
         )
+        capabilities.pop("passthru_devices", None)
         return capabilities
 
     def apply(self, operations, rotations=None, **kwargs):
diff --git a/tests/test_apply.py b/tests/test_apply.py
index 28fa6a78c..57c2ea84d 100644
--- a/tests/test_apply.py
+++ b/tests/test_apply.py
@@ -454,6 +454,29 @@ def test_load_default_qubit_device(self):
         assert dev.analytic
         assert dev.short_name == "lightning.qubit"
 
+    def test_no_backprop(self):
+        """Test that lightning.qubit does not support the backprop
+        differentiation method."""
+
+        dev = qml.device("lightning.qubit", wires=2)
+        def circuit():
+            """Simple quantum function."""
+            return qml.expval(qml.PauliZ(0))
+
+        with pytest.raises(qml.QuantumFunctionError):
+            qml.QNode(circuit, dev, diff_method="backprop")
+
+    def test_best_gets_lightning(self):
+        """Test that the best differentiation method returns lightning
+        qubit."""
+        dev = qml.device("lightning.qubit", wires=2)
+        def circuit():
+            """Simple quantum function."""
+            return qml.expval(qml.PauliZ(0))
+
+        qnode = qml.QNode(circuit, dev, diff_method="best")
+        assert isinstance(qnode.device, LightningQubit)
+
     def test_args(self):
         """Test that the plugin requires correct arguments"""