QPY serialization fails for circuits with IfElse instructions

Environment

• Qiskit Terra version: 0.20.0.dev0+01913f4
• Python version: 3.7
• Operating system: Mac OSX 11.6.1

What is happening?

QPY serialization fails when attempting to serialize a circuit containing an IfElseOp.

How can we reproduce the issue?

import qiskit as qk

qc = qk.QuantumCircuit(2, 2)
qc.h(0)
qc.measure(0, 0)

with qc.if_test((qc.clbits[0], True)):
    qc.x(1)
    
qc.measure(1,1)

from qiskit.circuit import qpy_serialization as qpy
import tempfile

with tempfile.TemporaryFile() as fp:
    qpy.dump(qc, fp)
    out = qpy.load(fp)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/var/folders/97/vkqk0hs17r5dnlc8dv5nmfxh0000gn/T/ipykernel_34451/736355381.py in <module>
      3 
      4 with tempfile.TemporaryFile() as fp:
----> 5     qpy.dump(qc, fp)
      6     out = qpy.load(fp)

~/q/qiskit-terra/qiskit/circuit/qpy_serialization.py in dump(circuits, file_obj)
   1423     file_obj.write(header)
   1424     for circuit in circuits:
-> 1425         _write_circuit(file_obj, circuit)
   1426 
   1427 

~/q/qiskit-terra/qiskit/circuit/qpy_serialization.py in _write_circuit(file_obj, circuit)
   1493     index_map["c"] = clbit_indices
   1494     for instruction in circuit.data:
-> 1495         _write_instruction(instruction_buffer, instruction, custom_instructions, index_map)
   1496     file_obj.write(struct.pack(CUSTOM_DEFINITION_HEADER_PACK, len(custom_instructions)))
   1497 

~/q/qiskit-terra/qiskit/circuit/qpy_serialization.py in _write_instruction(file_obj, instruction_tuple, custom_instructions, index_map)
   1223         else:
   1224             raise TypeError(
-> 1225                 f"Invalid parameter type {instruction_tuple[0]} for gate {type(param)},"
   1226             )
   1227         instruction_param_raw = struct.pack(INSTRUCTION_PARAM_PACK, type_key.encode("utf8"), size)

TypeError: Invalid parameter type Instruction(name='if_else', num_qubits=1, num_clbits=1, params=[<qiskit.circuit.quantumcircuit.QuantumCircuit object at 0x13c510750>, None]) for gate <class 'qiskit.circuit.quantumcircuit.QuantumCircuit'>,

What should happen?

The circuit should be able to be serialized and de-serialized.

Any suggestions?

No response

[mtreinish]

Right, I just meant we have a place already for this kind of thing where we have to do that kind of custom deserialization construction. Since we had to do it for Barrier (which does Barrier(num_qubits) which isn’t a parameter) and Initialize (which does Initialize(params) instead of Initialize(*params). So for 0.19.4 I think if we do something like:

if gate_name in {"IfElseOp", "WhileLoopOp"}:
    gate_class(condition, *params)

around here: https://github.com/Qiskit/qiskit-terra/blob/main/qiskit/circuit/qpy_serialization.py#L986-L992 that fits in with an existing pattern and won’t be too intrusive and seems like the simple path for a backport fix.

As for None, that’s a good call we don’t support that yet either. That’s trivial to add as a supported param type, we can add that at the same time we do QuantumCircuit

[jakelishman]

Having the condition is fine, the problem is that the constructor of IfElseOp and WhileLoopOp take condition as their first positional argument, and don’t put condition in the params attribute. The condition is the same tuple, but you do something like

my_if = IfElseOp((creg, 1), true_body, false_body)
assert len(my_if.params) == 2
assert my_if.params[0] is true_body
assert my_if.params[1] is false_body

We can do it, but it’ll need special handling for IfElseOp and WhileLoopOp when reconstructing the instances, to pass the condition positionally first (they error if there’s no condition).

Also, can qpy currently serialise a parameter of None? IfElseOp can have None as its second parameter if there isn’t a false body.