[BUGFIX] Removing measurement process hash/equality warnings (#4498)

* Adding pytest error

* Fixed errors

* Update changelog

* linting

* Fixed utils

* Fixed error

* Update tests/ops/functions/test_equal.py

Co-authored-by: Matthew Silverman <[email protected]>

* Change versions (#4499)

* Trigger CI

* Trigger CI

---------

Co-authored-by: Matthew Silverman <[email protected]>

doc/releases/changelog-0.32.0.md

@@ -345,6 +345,7 @@ array([False, False])
   [(#4144)](https://github.com/PennyLaneAI/pennylane/pull/4144)
   [(#4454)](https://github.com/PennyLaneAI/pennylane/pull/4454)
   [(#4489)](https://github.com/PennyLaneAI/pennylane/pull/4489)
+  [(#4498)](https://github.com/PennyLaneAI/pennylane/pull/4498)
 
 * The `sampler_seed` argument of `qml.gradients.spsa_grad` has been deprecated, along with a bug
   fix of the seed-setting behaviour.

pennylane/measurements/mid_measure.py

@@ -16,6 +16,7 @@
 """
 import uuid
 from typing import Generic, TypeVar, Optional
+import warnings
 
 import pennylane as qml
 import pennylane.numpy as np
@@ -255,16 +256,35 @@ def _apply(self, fn):
     def _merge(self, other: "MeasurementValue"):
         """Merge two measurement values"""
 
-        # create a new merged list with no duplicates and in lexical ordering
-        merged_measurements = list(set(self.measurements).union(set(other.measurements)))
+        with warnings.catch_warnings():
+            # Using a filter because the new behaviour of MP hash will be valid here
+            warnings.filterwarnings(
+                "ignore",
+                message="The behaviour of measurement process hashing",
+                category=UserWarning,
+            )
+            # create a new merged list with no duplicates and in lexical ordering
+            merged_measurements = list(set(self.measurements).union(set(other.measurements)))
+
         merged_measurements.sort(key=lambda m: m.id)
 
         # create a new function that selects the correct indices for each sub function
         def merged_fn(*x):
-            sub_args_1 = (x[i] for i in [merged_measurements.index(m) for m in self.measurements])
-            out_1 = self.processing_fn(*sub_args_1)
+            with warnings.catch_warnings():
+                # Using a filter because the new behaviour of MP equality will be valid here
+                warnings.filterwarnings(
+                    "ignore",
+                    message="The behaviour of measurement process equality",
+                    category=UserWarning,
+                )
+                sub_args_1 = (
+                    x[i] for i in [merged_measurements.index(m) for m in self.measurements]
+                )
+                sub_args_2 = (
+                    x[i] for i in [merged_measurements.index(m) for m in other.measurements]
+                )
 
-            sub_args_2 = (x[i] for i in [merged_measurements.index(m) for m in other.measurements])
+            out_1 = self.processing_fn(*sub_args_1)
             out_2 = other.processing_fn(*sub_args_2)
 
             return out_1, out_2

pennylane/ops/functions/equal.py

@@ -21,6 +21,7 @@
 import pennylane as qml
 from pennylane.measurements import MeasurementProcess
 from pennylane.measurements.classical_shadow import ShadowExpvalMP
+from pennylane.measurements.mid_measure import MidMeasureMP
 from pennylane.measurements.mutual_info import MutualInfoMP
 from pennylane.measurements.vn_entropy import VnEntropyMP
 from pennylane.pulse.parametrized_evolution import ParametrizedEvolution
@@ -355,6 +356,19 @@ def _equal_measurements(
     return False
 
 
+@_equal.register
+# pylint: disable=unused-argument
+def _equal_mid_measure(
+    op1: MidMeasureMP,
+    op2: MidMeasureMP,
+    check_interface=True,
+    check_trainability=True,
+    rtol=1e-5,
+    atol=1e-9,
+):
+    return op1.wires == op2.wires and op1.id == op2.id and op1.reset == op2.reset
+
+
 @_equal.register
 # pylint: disable=unused-argument
 def _(op1: VnEntropyMP, op2: VnEntropyMP, **kwargs):

pennylane/transforms/hamiltonian_expand.py

@@ -328,15 +328,28 @@ def sum_expand(tape: QuantumTape, group=True):
     idxs_coeffs = list(idxs_coeffs_dict.values())
 
     # Create the tapes, group observables if group==True
+    # pylint: disable=too-many-nested-blocks
     if group:
         m_groups = _group_measurements(measurements)
         # Update ``idxs_coeffs`` list such that it tracks the new ``m_groups`` list of lists
         tmp_idxs = []
         for m_group in m_groups:
             if len(m_group) == 1:
-                tmp_idxs.append(idxs_coeffs[measurements.index(m_group[0])])
+                # pylint: disable=undefined-loop-variable
+                for i, m in enumerate(measurements):
+                    if m is m_group[0]:
+                        break
+                tmp_idxs.append(idxs_coeffs[i])
             else:
-                tmp_idxs.append([idxs_coeffs[measurements.index(m)] for m in m_group])
+                inds = []
+                for mp in m_group:
+                    # pylint: disable=undefined-loop-variable
+                    for i, m in enumerate(measurements):
+                        if m is mp:
+                            break
+                    inds.append(idxs_coeffs[i])
+                tmp_idxs.append(inds)
+
         idxs_coeffs = tmp_idxs
         qscripts = [
             QuantumScript(ops=tape.operations, measurements=m_group, shots=tape.shots)

tests/ops/functions/test_equal.py

@@ -1236,6 +1236,26 @@ def test_observables_equal_but_wire_order_not(self):
         o2 = qml.prod(z0, x1)
         assert qml.equal(qml.expval(o1), qml.expval(o2))
 
+    def test_mid_measure(self):
+        """Test that `MidMeasureMP`s are equal only if their wires
+        an id are equal and their `reset` attribute match."""
+        mp = qml.measurements.MidMeasureMP(wires=qml.wires.Wires([0, 1]), reset=True, id="test_id")
+
+        mp1 = qml.measurements.MidMeasureMP(wires=qml.wires.Wires([1, 0]), reset=True, id="test_id")
+        mp2 = qml.measurements.MidMeasureMP(
+            wires=qml.wires.Wires([0, 1]), reset=False, id="test_id"
+        )
+        mp3 = qml.measurements.MidMeasureMP(wires=qml.wires.Wires([0, 1]), reset=True, id="foo")
+
+        assert not qml.equal(mp, mp1)
+        assert not qml.equal(mp, mp2)
+        assert not qml.equal(mp, mp3)
+
+        assert qml.equal(
+            mp,
+            qml.measurements.MidMeasureMP(wires=qml.wires.Wires([0, 1]), reset=True, id="test_id"),
+        )
+
 
 class TestObservablesComparisons:
     """Tests comparisons between Hamiltonians, Tensors and PauliX/Y/Z operators"""

tests/pytest.ini

@@ -21,3 +21,4 @@ filterwarnings =
     ignore:Call to deprecated create function:DeprecationWarning
     ignore:the imp module is deprecated:DeprecationWarning
     error:The behaviour of operator:UserWarning
+    error:The behaviour of measurement process:UserWarning