Mutable SingletonGates are modified when passed through pickle serialization and deserialization

[wshanks]

Environment

• Qiskit Terra version: e5420d2 (main branch with SingletonGate addition)
• Python version: 3.10
• Operating system: Fedora Linux 38

What is happening?

pickle.load's default procedure is to call cls.__new__(cls) to generate an object. Since SingletonGate overrides __new__ and behaves differently based on the arguments to __new__, pickle.load will generate an immutable object even when the pickled object was mutable (since SingletonGate(*args, **kwargs) pass the arguments to __new__ and __init__ but pickle.load does not by default).

How can we reproduce the issue?

from qiskit.circuit.library import SXGate
from pickle import loads, dumps
sx = SXGate()
sx50 = SXGate(duration=50)
new_sx50 = loads(dumps(sx))
print(f"sx with duration has same duration after pickle round trip: {sx50.duration == new_sx50.duration = }")
print(f"sx with duration is the immutable gate after pickle roundtrip: {sx is new_sx50 = }")

This gives:

sx with duration has same duration after pickle round trip: sx50.duration == new_sx50.duration = False
sx with duration is the immutable gate after pickle roundtrip: sx is new_sx50 = True

What should happen?

A mutable SingletonGate should pass through pickle and still have the same duration, condition, label, and unit.

Any suggestions?

This is fixed in #10871. I Just wanted to make an issue about it to refer to.

This issue was discovered by a test in qiskit-experiments that used a conditional gate. When the circuits using the conditonal gate were transpiled on Linux, the circuits were passed through pickle as part of Python's multiprocessing and the conditional gate lost its condition.

[wshanks]

Closed by #10871