Add deprecation warning to from_qasm

doc/development/deprecations.rst

@@ -21,6 +21,14 @@ Pending deprecations
   - Deprecated in v0.37
   - Will be removed in v0.38
 
+* ``qml.from_qasm`` will no longer remove measurements from the QASM code. Calling ``qml.from_qasm``
+  without specifying ``measurements`` will raise a deprecation warning in v0.37, and in v0.38,
+  the default behaviour will be changed to keeping measurements. Use ``measurements=[]`` to remove
+  measurements from the original circuit.
+
+  - Deprecated in v0.37
+  - Default behaviour will be changed in v0.38
+
 New operator arithmetic deprecations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

doc/releases/changelog-dev.md

@@ -405,6 +405,9 @@
 * `qml.transforms.map_batch_transform` is deprecated, since a transform can be applied directly to a batch of tapes.
   [(#5676)](https://github.com/PennyLaneAI/pennylane/pull/5676)
 
+* The default behaviour of `qml.from_qasm()` to remove measurements in the QASM code is deprecated. Use `measurements=[]` to keep this behaviour or `measurements=None` to keep the measurements from the QASM code.
+  [(#5882)](https://github.com/PennyLaneAI/pennylane/pull/5882)
+
 <h3>Documentation 📝</h3>
 
 * Move information about mid-circuit measurements from the measurements quickstart page to its own

pennylane/io.py

@@ -15,10 +15,13 @@
 This module contains functions to load circuits from other frameworks as
 PennyLane templates.
 """
+import warnings
 from collections import defaultdict
 from importlib import metadata
 from sys import version_info
 
+import pennylane as qml
+
 # Error message to show when the PennyLane-Qiskit plugin is required but missing.
 _MISSING_QISKIT_PLUGIN_MESSAGE = (
     "Conversion from Qiskit requires the PennyLane-Qiskit plugin. "
@@ -407,7 +410,7 @@ def from_qiskit_op(qiskit_op, params=None, wires=None):
         raise RuntimeError(_MISSING_QISKIT_PLUGIN_MESSAGE) from e
 
 
-def from_qasm(quantum_circuit: str, measurements=None):
+def from_qasm(quantum_circuit: str, measurements=False):
     """Loads quantum circuits from a QASM string using the converter in the
     PennyLane-Qiskit plugin.
 
@@ -436,8 +439,16 @@ def from_qasm(quantum_circuit: str, measurements=None):
         that will **override** any terminal measurements present in the QASM code,
         so that they are not performed before the operations specified in ``measurements``.
 
-    If the existing QASM code already contains measurements, ``from_qasm``
-    will return those measurements, provided that they are not overriden as shown above.
+    By default, ``from_qasm`` will remove any measurements that are present in the QASM code.
+    If the QASM code contains measurements, set ``measurements=None`` to keep them in the
+    output of ``from_qasm``.
+
+    .. warning::
+
+        The current default behaviour of removing measurements in the QASM code is deprecated
+        and will be changed in a future release. Starting in version ``0.38``, ``from_qasm``
+        will keep the measurements from the QASM code by default. To remove all measurements,
+        set ``measurements=[]`` which overrides the existing measurements with an empty list.
 
     Mid-circuit measurements inside the QASM code can also be interpreted.
 
@@ -485,12 +496,23 @@ def circuit():
         quantum_circuit (str): a QASM string containing a valid quantum circuit
         measurements (None | MeasurementProcess | list[MeasurementProcess]): an optional PennyLane
             measurement or list of PennyLane measurements that overrides any terminal measurements
-            that may be present in the input circuit
+            that may be present in the input circuit. If set to ``None``, existing measurements
+            in the input circuit will be used.
 
     Returns:
         function: the PennyLane template created based on the QASM string
+
     """
     plugin_converter = plugin_converters["qasm"].load()
+    if measurements is False:
+        warnings.warn(
+            "The current default behaviour of removing measurements in the QASM code is "
+            "deprecated. Set measurements=None to keep the existing measurements in the QASM "
+            "code or set measurements=[] to remove them from the returned circuit. Starting "
+            "in version 0.38, measurements=None will be the new default.",
+            qml.PennyLaneDeprecationWarning,
+        )
+        measurements = []
     return plugin_converter(quantum_circuit, measurements=measurements)
 
 

tests/test_io.py

@@ -114,7 +114,6 @@ def test_qiskit_converter_load_fails(self, monkeypatch, method, entry_point_name
         [
             (qml.from_qiskit, "qiskit"),
             (qml.from_qiskit_op, "qiskit_op"),
-            (qml.from_qasm, "qasm"),
             (qml.from_pyquil, "pyquil_program"),
             (qml.from_quil, "quil"),
             (qml.from_quil_file, "quil_file"),
@@ -135,11 +134,38 @@ def test_convenience_functions(self, method, entry_point_name, mock_plugin_conve
             if mock_plugin_converters[plugin_converter].called:
                 raise RuntimeError(f"The other plugin converter {plugin_converter} was called.")
 
+    def test_from_qasm(self, mock_plugin_converters):
+        """Tests that the correct entry point is called for from_qasm."""
+
+        qml.from_qasm("Test", measurements=None)
+        assert mock_plugin_converters["qasm"].called
+        assert mock_plugin_converters["qasm"].last_args == ("Test",)
+
+        for plugin_converter in mock_plugin_converters:
+            if mock_plugin_converters[plugin_converter].called and plugin_converter != "qasm":
+                raise RuntimeError(f"The other plugin converter {plugin_converter} was called.")
+
+    def test_from_qasm_deprecated(self, mock_plugin_converters):
+        """Tests that the current default behaviour of from_qasm is deprecated."""
+
+        with pytest.warns(qml.PennyLaneDeprecationWarning, match="The current default behaviour"):
+            qml.from_qasm("Test")
+
+        called_args, called_kwargs = mock_plugin_converters["qasm"].call_args
+        assert called_args == ("Test",)
+        assert called_kwargs == {"measurements": []}
+
     @pytest.mark.parametrize(
         "method, entry_point_name, args, kwargs",
         [
             (qml.from_qiskit, "qiskit", ("Circuit",), {"measurements": []}),
             (qml.from_qiskit_op, "qiskit_op", ("Op",), {"params": [1, 2], "wires": [3, 4]}),
+            (
+                qml.from_qasm,
+                "qasm",
+                ("Circuit",),
+                {"measurements": []},
+            ),
         ],
     )
     def test_convenience_function_arguments(