python - pickle 和复制持久对象的类？

Question

我正在尝试为只读对象编写一个类，该类不会使用 copy 模块进行真正复制，并且当它被 pickle 以在进程之间传输时，每个进程将维护最多只有一个副本，无论它作为"new"对象传递多少次。已经有类似的东西了吗？

Answer 1

我试图实现这一点。 @Alex Martelli 和其他任何人，请给我评论/改进。我认为这最终会出现在 GitHub 上。

"""
todo: need to lock library to avoid thread trouble?

todo: need to raise an exception if we're getting pickled with
an old protocol?

todo: make it polite to other classes that use __new__. Therefore, should
probably work not only when there is only one item in the *args passed to new.

"""

import uuid
import weakref

library = weakref.WeakValueDictionary()

class UuidToken(object):
    def __init__(self, uuid):
        self.uuid = uuid


class PersistentReadOnlyObject(object):
    def __new__(cls, *args, **kwargs):
        if len(args)==1 and len(kwargs)==0 and isinstance(args[0], UuidToken):
            received_uuid = args[0].uuid
        else:
            received_uuid = None

        if received_uuid:
            # This section is for when we are called at unpickling time
            thing = library.pop(received_uuid, None)
            if thing:
                thing._PersistentReadOnlyObject__skip_setstate = True
                return thing
            else: # This object does not exist in our library yet; Let's add it
                new_args = args[1:]
                thing = super(PersistentReadOnlyObject, cls).__new__(cls,
                                                                     *new_args,
                                                                     **kwargs)
                thing._PersistentReadOnlyObject__uuid = received_uuid
                library[received_uuid] = thing
                return thing

        else:
            # This section is for when we are called at normal creation time
            thing = super(PersistentReadOnlyObject, cls).__new__(cls, *args,
                                                                 **kwargs)
            new_uuid = uuid.uuid4()
            thing._PersistentReadOnlyObject__uuid = new_uuid
            library[new_uuid] = thing
            return thing

    def __getstate__(self):
        my_dict = dict(self.__dict__)
        del my_dict["_PersistentReadOnlyObject__uuid"]
        return my_dict

    def __getnewargs__(self):
        return (UuidToken(self._PersistentReadOnlyObject__uuid),)

    def __setstate__(self, state):
        if self.__dict__.pop("_PersistentReadOnlyObject__skip_setstate", None):
            return
        else:
            self.__dict__.update(state)

    def __deepcopy__(self, memo):
        return self

    def __copy__(self):
        return self

# --------------------------------------------------------------
"""
From here on it's just testing stuff; will be moved to another file.
"""


def play_around(queue, thing):
    import copy
    queue.put((thing, copy.deepcopy(thing),))

class Booboo(PersistentReadOnlyObject):
    def __init__(self):
        self.number = random.random()

if __name__ == "__main__":

    import multiprocessing
    import random
    import copy

    def same(a, b):
        return (a is b) and (a == b) and (id(a) == id(b)) and \
               (a.number == b.number)

    a = Booboo()
    b = copy.copy(a)
    c = copy.deepcopy(a)
    assert same(a, b) and same(b, c)

    my_queue = multiprocessing.Queue()
    process = multiprocessing.Process(target = play_around,
                                      args=(my_queue, a,))
    process.start()
    process.join()
    things = my_queue.get()
    for thing in things:
        assert same(thing, a) and same(thing, b) and same(thing, c)
    print("all cool!")