pymoo Multiprocessing process based parallelization fails on 2nd generation of NSGA-II -> pymoo 0.6.0

I am trying to execute the following moo using an NSGA-II algorithm and parallelized using the multiprocessing starmap. This problem previously worked with the 0.5.0 implementation of parallelization.


pool = multiprocessing.Pool(n_proccess)
runner = StarmapParallelization(pool.starmap)

 problem = myproblem.(
    elementwise_runner=runner,
)

algorithm = NSGA2(
    pop_size= 10
    n_offsprings=2
    sampling=MixedVariableSampling(),
    mating=MixedVariableMating(
        eliminate_duplicates=MixedVariableDuplicateElimination()
    ),
    eliminate_duplicates=MixedVariableDuplicateElimination(),
)

results = minimize(
    problem,
    algorithm,
    termination=("n_gen", 10),
    seed=1,
    save_history=True,
    verbose=True,
)

I receive the following error:

Traceback (most recent call last):
  File "c:/Users/first.last/Documents/mypkg/src/my_module/__main__.py", line 128, in optimizer
    results = minimize(
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\optimize.py", line 67, in minimize
    res = algorithm.run()
  File "C:\Users\first.last\Documents\mypkg Gears\.venv\lib\site-packages\pymoo\core\algorithm.py", line 141, in run
    self.next()
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\algorithm.py", line 161, in next
    self.evaluator.eval(self.problem, infills, algorithm=self)
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\evaluator.py", line 84, in eval
    self._eval(problem, pop[I], evaluate_values_of, **kwargs)
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\evaluator.py", line 105, in _eval
    out = problem.evaluate(
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\problem.py", line 190, in evaluate
    _out = self.do(X, return_values_of, *args, **kwargs)
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\problem.py", line 230, in do
    self._evaluate_elementwise(X, out, *args, **kwargs)
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\problem.py", line 248, in _evaluate_elementwise
    elems = self.elementwise_runner(f, X)
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\problem.py", line 34, in __call__
    return list(self.starmap(f, [[x] for x in X]))
  File "C:\Users\first.last\AppData\Local\Programs\Python\Python38\lib\multiprocessing\pool.py", line 372, in starmap
    return self._map_async(func, iterable, starmapstar, chunksize).get()
  File "C:\Users\first.last\AppData\Local\Programs\Python\Python38\lib\multiprocessing\pool.py", line 771, in get
    raise self._value
  File "C:\Users\first.last\AppData\Local\Programs\Python\Python38\lib\multiprocessing\pool.py", line 537, in _handle_tasks
    put(task)
  File "C:\Users\first.last\AppData\Local\Programs\Python\Python38\lib\multiprocessing\connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "C:\Users\first.last\AppData\Local\Programs\Python\Python38\lib\multiprocessing\reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
  File "C:\Users\first.last\Documents\mypkg\.venv\lib\site-packages\pymoo\core\problem.py", line 38, in __getstate__
    state.pop("starmap")
KeyError: 'starmap'

The error can be cleared by modifying this code in the pymoo/core/problem.py file

from:

class StarmapParallelization:
    def __init__(self, starmap) -> None:
        super().__init__()
        self.starmap = starmap

    def __call__(self, f, X):
        return list(self.starmap(f, [[x] for x in X]))

    def __getstate__(self):
        state = self.__dict__.copy()
        state.pop("starmap")
        return state

to:

class StarmapParallelization:
    def __init__(self, starmap) -> None:
        super().__init__()
        self.starmap = starmap

    def __call__(self, f, X):
        return list(self.starmap(f, [[x] for x in X]))

    def __getstate__(self):
        state = self.__dict__.copy()
        state.pop("starmap",None)
        return state

This provides a default value fro the state.pop() call in the getstate method