I came accross this looking for a way to skip a non picklable attribute and based on JacobP's answer I'm using the below. It uses the same reference to skipped as the original instance.

def __deepcopy__(self, memo):
    cls = self.__class__
    obj = cls.__new__(cls)
    memo[id(self)] = obj
    for k, v in self.__dict__.items():
        if k not in ['skipped']:
            v = copy.deepcopy(v, memo)
        setattr(obj, k, v)
    return obj