add warning about applying a queued operator (#4311)

* add warning about applying a queued operator * move warning into warning block
PennyLaneAI · Jul 4, 2023 · 454134e · 454134e
1 parent 78f1199
commit 454134e
Showing 1 changed file with 16 additions and 0 deletions.
diff --git a/pennylane/queuing.py b/pennylane/queuing.py
@@ -459,6 +459,22 @@ def circuit(x):
  >>> print(qml.draw(circuit)(0.6))
  0: ──RX(0.4)──RY(0.6)──RX(0.4)──┤ ⟨Z⟩
 
+ .. warning::
+
+ If you use ``apply`` on an operator that has already been queued, it will
+ be queued for a second time. For example:
+
+ .. code-block:: python
+
+ @qml.qnode(dev)
+ def circuit():
+ op = qml.Hadamard(0)
+ qml.apply(op)
+ return qml.expval(qml.PauliZ(0))
+
+ >>> print(qml.draw(circuit)())
+ 0: ──H──H─┤ <Z>
+
  .. details::
  :title: Usage Details