Parameterized circuits can no longer be loaded with qpy.

[Guillermo-Mijares-Vilarino]

Environment

• Qiskit version: 1.3.2
• Python version: 3.12.7
• Operating system: macOS Sequoia 15.1.1

What is happening?

After upgrading qiskit from 1.2.4 to 1.3.2, qpy started failing to load parameterized circuits.

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[2], line 10
      7     qpy.dump(fm, file)
      9 with open(\"circuit.qpy\", \"rb\") as file:
---> 10     new_fm = qpy.load(file)[0]

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/interface.py:370, in load(file_obj, metadata_deserializer)
    367 programs = []
    368 for _ in range(data.num_programs):
    369     programs.append(
--> 370         loader(
    371             file_obj,
    372             data.qpy_version,
    373             metadata_deserializer=metadata_deserializer,
    374             use_symengine=use_symengine,
    375         )
    376     )
    377 return programs

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/circuits.py:1441, in read_circuit(file_obj, version, metadata_deserializer, use_symengine)
   1439 for declaration in var_segments[type_keys.ExprVarDeclaration.LOCAL]:
   1440     circ.add_uninitialized_var(declaration)
-> 1441 custom_operations = _read_custom_operations(file_obj, version, vectors)
   1442 for _instruction in range(num_instructions):
   1443     _read_instruction(
   1444         file_obj,
   1445         circ,
   (...)
   1451         standalone_var_indices,
   1452     )

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/circuits.py:617, in _read_custom_operations(file_obj, version, vectors)
    615 def_binary = file_obj.read(data.size)
    616 if version < 3 or not name.startswith(r\"###PauliEvolutionGate_\"):
--> 617     definition_circuit = common.data_from_binary(
    618         def_binary, read_circuit, version=version
    619     )
    620 elif name.startswith(r\"###PauliEvolutionGate_\"):
    621     definition_circuit = common.data_from_binary(
    622         def_binary, _read_pauli_evolution_gate, version=version, vectors=vectors
    623     )

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/common.py:267, in data_from_binary(binary_data, deserializer, **kwargs)
    265 with io.BytesIO(binary_data) as container:
    266     container.seek(0)
--> 267     obj = deserializer(container, **kwargs)
    268 return obj

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/circuits.py:1443, in read_circuit(file_obj, version, metadata_deserializer, use_symengine)
   1441 custom_operations = _read_custom_operations(file_obj, version, vectors)
   1442 for _instruction in range(num_instructions):
-> 1443     _read_instruction(
   1444         file_obj,
   1445         circ,
   1446         out_registers,
   1447         custom_operations,
   1448         version,
   1449         vectors,
   1450         use_symengine,
   1451         standalone_var_indices,
   1452     )
   1454 # Read calibrations
   1455 if version >= 5:

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/circuits.py:297, in _read_instruction(file_obj, circuit, registers, custom_operations, version, vectors, use_symengine, standalone_vars)
    295 for _param in range(instruction.num_parameters):
    296     type_key, data_bytes = common.read_generic_typed_data(file_obj)
--> 297     param = _loads_instruction_parameter(
    298         type_key,
    299         data_bytes,
    300         version,
    301         vectors,
    302         registers,
    303         circuit,
    304         use_symengine,
    305         standalone_vars,
    306     )
    307     params.append(param)
    309 # Load Gate object

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/circuits.py:201, in _loads_instruction_parameter(type_key, data_bytes, version, vectors, registers, circuit, use_symengine, standalone_vars)
    199 else:
    200     clbits = circuit.clbits if circuit is not None else ()
--> 201     param = value.loads_value(
    202         type_key,
    203         data_bytes,
    204         version,
    205         vectors,
    206         clbits=clbits,
    207         cregs=registers[\"c\"],
    208         use_symengine=use_symengine,
    209         standalone_vars=standalone_vars,
    210     )
    212 return param

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/value.py:971, in loads_value(type_key, binary_data, version, vectors, clbits, cregs, use_symengine, standalone_vars)
    964         return common.data_from_binary(
    965             binary_data,
    966             _read_parameter_expression_v3,
    967             vectors=vectors,
    968             use_symengine=use_symengine,
    969         )
    970     else:
--> 971         return common.data_from_binary(
    972             binary_data, _read_parameter_expression_v13, vectors=vectors, version=version
    973         )
    974 if type_key == type_keys.Value.EXPRESSION:
    975     return common.data_from_binary(
    976         binary_data,
    977         _read_expr,
   (...)
    980         standalone_vars=standalone_vars,
    981     )

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/common.py:267, in data_from_binary(binary_data, deserializer, **kwargs)
    265 with io.BytesIO(binary_data) as container:
    266     container.seek(0)
--> 267     obj = deserializer(container, **kwargs)
    268 return obj

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/value.py:518, in _read_parameter_expression_v13(file_obj, vectors, version)
    516     symbol = _read_parameter(file_obj)
    517 elif symbol_key == type_keys.Value.PARAMETER_VECTOR:
--> 518     symbol = _read_parameter_vec(file_obj, vectors)
    519 else:
    520     raise exceptions.QpyError(f\"Invalid parameter expression map type: {symbol_key}\")

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/qpy/binary_io/value.py:402, in _read_parameter_vec(file_obj, vectors)
    400     vectors[name] = (ParameterVector(name, data.vector_size), set())
    401 vector = vectors[name][0]
--> 402 if vector[data.index].uuid != param_uuid:
    403     vectors[name][1].add(data.index)
    404     vector._params[data.index] = ParameterVectorElement(vector, data.index, uuid=param_uuid)

File ~/miniconda3/envs/pqf_function/lib/python3.12/site-packages/qiskit/circuit/parametervector.py:78, in ParameterVector.__getitem__(self, key)
     77 def __getitem__(self, key):
---> 78     return self.params[key]

IndexError: list index out of range

How can we reproduce the issue?

from qiskit import qpy
from qiskit.circuit.library import PauliFeatureMap

qc = PauliFeatureMap(feature_dimension=5, reps=1)

with open("circuit.qpy", "wb") as fd:
    qpy.dump(qc, fd)

with open("circuit.qpy", "rb") as fd:
    new_qc = qpy.load(fd)[0]

What should happen?

The circuit should be loaded without raising an error, as in qiskit<1.3.0.

Any suggestions?

No response

[ElePT]

Thanks for reporting this bug! I did a quick investigation and I think the issue is related to the "special" class of circuit of the PauliFeatureMap (which is not a QuantumCircuit but a BlueprintCircuit). We will look more into it, but as an intermediate solution, you can use the new constructor for the pauli feature map which does produce a QuantumCircuit:

from qiskit import qpy
from qiskit.circuit.library import pauli_feature_map

qc = pauli_feature_map(feature_dimension=5, reps=1)

with open("circuit.qpy", "wb") as fd:
    qpy.dump(qc, fd)

with open("circuit.qpy", "rb") as fd:
    new_qc = qpy.load(fd)[0]