QPY fails to roundtrip expressions with cancelled-out variables #15355
Description
Environment
- Qiskit version: 2.2.3
- Python version: 3.13
- Operating system: macOS
What is happening?
QPY roundtrips can fail on certain ParameterExpressions where a variable is symbolically eliminated without having been bound.
How can we reproduce the issue?
import io
from qiskit import qpy, QuantumCircuit
from qiskit.circuit import Parameter
x = Parameter("x")
# Using either of these triggers the same bug.
bad = [0 * x, x - x]
qc = QuantumCircuit(1)
qc.rz(bad[0], 0)
with io.BytesIO() as fptr:
qpy.dump(qc, fptr)
fptr.seek(0)
qpy.load(fptr)[0]File ~/code/qiskit/terra/qiskit/qpy/interface.py:430, in load(file_obj, metadata_deserializer, annotation_factories)
426 if data.qpy_version >= 16:
427 # Deserialize each program using their byte offsets
428 file_obj.seek(program_offsets[i])
429 programs.append(
--> 430 binary_io.read_circuit(
431 file_obj,
432 data.qpy_version,
433 metadata_deserializer=metadata_deserializer,
434 use_symengine=use_symengine,
435 annotation_factories=annotation_factories,
436 )
437 )
438 return programs
[... snip ...]
File ~/code/qiskit/terra/qiskit/qpy/binary_io/value.py:611, in _read_parameter_expression_v13(file_obj, vectors, version)
609 symbol_map[symbol] = value
610 with io.BytesIO(payload) as buf:
--> 611 return _read_parameter_expr_v13(buf, symbol_map, version, vectors)
File ~/code/qiskit/terra/qiskit/qpy/binary_io/value.py:707, in _read_parameter_expr_v13(buf, symbol_map, version, vectors)
704 stack.append(getattr(lhs, method_str)())
705 data = buf.read(formats.PARAM_EXPR_ELEM_V13_SIZE)
--> 707 return stack.pop()
IndexError: pop from empty list
What should happen?
Roundtrip should succeed.
Any suggestions?
I'd assume the bug actually happens in the QPY dump (not writing the relevant variable into the stack) but I haven't looked into it in detail.