def METHOD_NAME(observable: SparsePauliOp) -> tuple:
    """Private key function for SparsePauliOp.
    Args:
        observable: Input operator.
    Returns:
        Key for observables.
    """
    return (
        observable.paulis.z.tobytes(),
        observable.paulis.x.tobytes(),
        observable.paulis.phase.tobytes(),
        observable.coeffs.tobytes(),
    )