Do not retain and expose old elements of ParameterVector (#12561)

* Do not retain and expose old elements of `ParameterVector`

This fixes #12541 according to
#12545 (review).

* Fix test

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:]

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,
         )

test/python/circuit/test_parameters.py

@@ -1368,10 +1368,8 @@ def test_parametervector_resize(self):
         with self.subTest("enlargen"):
             vec.resize(3)
             self.assertEqual(len(vec), 3)
-            # ensure we still have the same instance not a copy with the same name
-            # this is crucial for adding parameters to circuits since we cannot use the same
-            # name if the instance is not the same
-            self.assertIs(element, vec[1])
+            # ensure we still have an element with the same uuid
+            self.assertEqual(element, vec[1])
             self.assertListEqual([param.name for param in vec], _paramvec_names("x", 3))
 
     def test_raise_if_sub_unknown_parameters(self):