turn QuantumScript.hash into a cached property

doc/releases/changelog-dev.md

@@ -8,6 +8,9 @@
 
 <h3>Improvements 🛠</h3>
 
+* `QuantumScript.hash` is now cached, leading to performance improvements.
+  [(#5919)](https://github.com/PennyLaneAI/pennylane/pull/5919)
+
 <h3>Breaking changes 💔</h3>
 
 <h3>Deprecations 👋</h3>
@@ -20,4 +23,5 @@
 
 This release contains contributions from (in alphabetical order):
 
-Yushao Chen.
+Yushao Chen,
+Christina Lee,

pennylane/tape/qscript.py

@@ -188,7 +188,7 @@ def __init__(
     def __repr__(self):
         return f"<{self.__class__.__name__}: wires={self.wires.tolist()}, params={self.num_params}>"
 
-    @property
+    @cached_property
     def hash(self):
         """int: returns an integer hash uniquely representing the quantum script"""
         fingerprint = []
@@ -378,11 +378,13 @@ def _update(self):
         """Update all internal metadata regarding processed operations and observables"""
         self._graph = None
         self._specs = None
+        self._trainable_params = None
 
         try:
             # Invalidate cached properties so they get recalculated
             del self.wires
             del self.par_info
+            del self.hash
         except AttributeError:
             pass
 

tests/tape/test_qscript.py

@@ -199,11 +199,16 @@ def test_cached_properties(self):
         ]
         m = [qml.expval(qml.Hermitian(2 * np.eye(2), wires=0))]
         qs = QuantumScript(ops, m)
+        assert qs.wires == qml.wires.Wires((0,))
+        assert isinstance(qs.par_info, list) and len(qs.par_info) > 0
+        old_hash = qs.hash
+        assert qs.trainable_params == [0, 1, 2, 3, 4, 5, 6, 7]
         qs._ops = []
         qs._measurements = []
         qs._update()
         assert qs.wires == qml.wires.Wires([])
         assert isinstance(qs.par_info, list) and len(qs.par_info) == 0
+        assert QuantumScript([], []).hash == qs.hash and qs.hash != old_hash
 
     # pylint: disable=unbalanced-tuple-unpacking
     def test_get_operation(self):