From 66a2a3356b7f4ada9a70866ed5a3d27219dc0085 Mon Sep 17 00:00:00 2001
From: Jim Garrison <garrison@ibm.com>
Date: Wed, 12 Jun 2024 20:35:39 -0400
Subject: [PATCH] Do not retain and expose old elements of `ParameterVector`

This fixes #12541 according to
https://github.com/Qiskit/qiskit/pull/12545#pullrequestreview-2114202382.
---
 qiskit/circuit/parametervector.py | 38 ++++++++++++++++++-------------
 qiskit/qpy/binary_io/value.py     |  2 +-
 2 files changed, 23 insertions(+), 17 deletions(-)

diff --git a/qiskit/circuit/parametervector.py b/qiskit/circuit/parametervector.py
index abc8a6f60ef7..151e3e7fea73 100644
--- a/qiskit/circuit/parametervector.py
+++ b/qiskit/circuit/parametervector.py
@@ -50,11 +50,10 @@ def __setstate__(self, state):
 class ParameterVector:
     """ParameterVector class to quickly generate lists of parameters."""
 
-    __slots__ = ("_name", "_params", "_size", "_root_uuid")
+    __slots__ = ("_name", "_params", "_root_uuid")
 
     def __init__(self, name, length=0):
         self._name = name
-        self._size = length
         self._root_uuid = uuid4()
         root_uuid_int = self._root_uuid.int
         self._params = [
@@ -76,32 +75,38 @@ def index(self, value):
         return self._params.index(value)
 
     def __getitem__(self, key):
-        if isinstance(key, slice):
-            start, stop, step = key.indices(self._size)
-            return self.params[start:stop:step]
-
-        if key > self._size:
-            raise IndexError(f"Index out of range: {key} > {self._size}")
         return self.params[key]
 
     def __iter__(self):
-        return iter(self.params[: self._size])
+        return iter(self.params)
 
     def __len__(self):
-        return self._size
+        return len(self._params)
 
     def __str__(self):
-        return f"{self.name}, {[str(item) for item in self.params[: self._size]]}"
+        return f"{self.name}, {[str(item) for item in self.params]}"
 
     def __repr__(self):
         return f"{self.__class__.__name__}(name={self.name}, length={len(self)})"
 
     def resize(self, length):
-        """Resize the parameter vector.
-
-        If necessary, new elements are generated. If length is smaller than before, the
-        previous elements are cached and not re-generated if the vector is enlarged again.
+        """Resize the parameter vector.  If necessary, new elements are generated.
+
+        Note that the UUID of each :class:`.Parameter` element will be generated
+        deterministically given the root UUID of the ``ParameterVector`` and the index
+        of the element.  In particular, if a ``ParameterVector`` is resized to
+        be smaller and then later resized to be larger, the UUID of the later
+        generated element at a given index will be the same as the UUID of the
+        previous element at that index.
         This is to ensure that the parameter instances do not change.
+
+        >>> from qiskit.circuit import ParameterVector
+        >>> pv = ParameterVector("theta", 20)
+        >>> elt_19 = pv[19]
+        >>> rv.resize(10)
+        >>> rv.resize(20)
+        >>> pv[19] == elt_19
+        True
         """
         if length > len(self._params):
             root_uuid_int = self._root_uuid.int
@@ -111,4 +116,5 @@ def resize(self, length):
                     for i in range(len(self._params), length)
                 ]
             )
-        self._size = length
+        else:
+            del self._params[length:]
diff --git a/qiskit/qpy/binary_io/value.py b/qiskit/qpy/binary_io/value.py
index fdad363867a3..c9f0f9af4798 100644
--- a/qiskit/qpy/binary_io/value.py
+++ b/qiskit/qpy/binary_io/value.py
@@ -45,7 +45,7 @@ def _write_parameter_vec(file_obj, obj):
         struct.pack(
             formats.PARAMETER_VECTOR_ELEMENT_PACK,
             len(name_bytes),
-            obj._vector._size,
+            len(obj._vector),
             obj.uuid.bytes,
             obj._index,
         )