From 76803d5919dcce926566a287b2229670f88e9cec Mon Sep 17 00:00:00 2001
From: "mergify[bot]" <37929162+mergify[bot]@users.noreply.github.com>
Date: Wed, 12 Jun 2024 07:49:59 +0200
Subject: [PATCH] Use hash of numeric value for bound parameter expressions
 (#12488) (#12548)

* Use hash of numeric value for bound parameter expressions

If a `ParameterExpression` has no unbound parameters, the underlying
bound value can be hashed instead of the tuple that accounts for the
symbolic expression. Doing this allows for the `ParameterExpression` to
match the hash for the numeric value it compares equal to.

Closes #12487

* Add release note

(cherry picked from commit bc685d3002915327f3070ef8914c6c1484084b57)

Co-authored-by: Will Shanks <willshanks@us.ibm.com>
---
 qiskit/circuit/parameterexpression.py                     | 3 +++
 .../notes/parameterexpression-hash-d2593ab1715aa42c.yaml  | 8 ++++++++
 test/python/circuit/test_parameters.py                    | 1 +
 3 files changed, 12 insertions(+)
 create mode 100644 releasenotes/notes/parameterexpression-hash-d2593ab1715aa42c.yaml

diff --git a/qiskit/circuit/parameterexpression.py b/qiskit/circuit/parameterexpression.py
index 4f6453f90f4c..b98a294b090f 100644
--- a/qiskit/circuit/parameterexpression.py
+++ b/qiskit/circuit/parameterexpression.py
@@ -439,6 +439,9 @@ def __int__(self):
             raise TypeError("could not cast expression to int") from exc
 
     def __hash__(self):
+        if not self._parameter_symbols:
+            # For fully bound expressions, fall back to the underlying value
+            return hash(self.numeric())
         return hash((self._parameter_keys, self._symbol_expr))
 
     def __copy__(self):
diff --git a/releasenotes/notes/parameterexpression-hash-d2593ab1715aa42c.yaml b/releasenotes/notes/parameterexpression-hash-d2593ab1715aa42c.yaml
new file mode 100644
index 000000000000..075de45b3b23
--- /dev/null
+++ b/releasenotes/notes/parameterexpression-hash-d2593ab1715aa42c.yaml
@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    :class:`.ParameterExpression` was updated so that fully bound instances
+    that compare equal to instances of Python's built-in numeric types (like
+    ``float`` and ``int``) also have hash values that match those of the other
+    instances. This change ensures that these types can be used interchangeably
+    as dictionary keys. See `#12488 <https://github.com/Qiskit/qiskit/pull/12488>`__.
diff --git a/test/python/circuit/test_parameters.py b/test/python/circuit/test_parameters.py
index 7bcc2cd35f3a..3681998c0638 100644
--- a/test/python/circuit/test_parameters.py
+++ b/test/python/circuit/test_parameters.py
@@ -1425,6 +1425,7 @@ def test_compare_to_value_when_bound(self):
         x = Parameter("x")
         bound_expr = x.bind({x: 2.3})
         self.assertEqual(bound_expr, 2.3)
+        self.assertEqual(hash(bound_expr), hash(2.3))
 
     def test_abs_function_when_bound(self):
         """Verify expression can be used with