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Cannot exclude members to pickle in metaclass

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Issue 类型
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描述清晰度
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技术栈
python
领域
tooling

调研方向

首先在提供的 SingletonMeta 示例中重现该故障，重点关注 test_singleton_pickle 和最后的 cloudpickle.dumps(test_singleton) 调用。阅读 cloudpickle 如何处理元类和 getstate 方法，然后验证实例可以在不尝试 pickle 元类的 RLock 的情况下进行转储和加载，同时保留所示的 assertions。

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描述

pickle issue

In the example below, I implemented __getstate__(cls) that returns empty dict
in order not to pickle cls.INSTANCES and cls.LOCK members of metaclass SingletonMeta.

But it seems that cloudpickle still tries to pickle those members
and fails to pickle since threading.RLock is not picklable.

import cloudpickle

from threading import RLock


class SingletonMeta(type):
    def __new__(cls, name, bases, namespace):
        return super().__new__(cls, name, bases, namespace)

    def __init__(cls, name, bases, namespace):
        super().__init__(name, bases, namespace)
        cls.INSTANCES = {}
        cls.LOCK = RLock()

    def __call__(cls, *args, **kwargs):
        key = cls.__getkey__(*args, **kwargs)
        key = tuple(key.items())
        if key not in cls.INSTANCES:
            with cls.LOCK:
                if key not in cls.INSTANCES:
                    cls.INSTANCES[key] = super().__call__(*args, **kwargs)
        instance = cls.INSTANCES[key]
        return instance

    def __getstate__(cls):
        # don't want to pickle cls.INSTANCES and cls.LOCK
        return {}

    def __setstate__(cls, state):
        pass


class SingletonClass(metaclass=SingletonMeta):
    def __init__(self, key):
        self.key = key
        self.transient = [self.key]

    def __getstate__(self):
        state = self.__dict__.copy()
        del state["transient"]
        return state

    def __setstate__(self, state):
        for name, value in state.items():
            setattr(self, name, value)
        self.transient = [self.key]

    @classmethod
    def __getkey__(cls, key):
        return {"key": key}


# singleton implementation works fine as long as metaclass = SingletonMeta
def test_singleton():
    singleton_instance1 = SingletonClass(0)
    singleton_instance2 = SingletonClass(0)
    singleton_instance3 = SingletonClass(1)
    assert singleton_instance1 is singleton_instance2
    assert singleton_instance1 is not singleton_instance3


# pickling instances works fine if metaclass != SingletonMeta
# but raises `TypeError: cannot pickle '_thread.RLock' object` if metaclass = SingletonMeta
def test_singleton_pickle():
    singleton_instance1 = SingletonClass(0)
    singleton_instance1_loaded = cloudpickle.loads(
        cloudpickle.dumps(singleton_instance1)
    )
    assert singleton_instance1 is not singleton_instance1_loaded
    assert singleton_instance1.key == singleton_instance1_loaded.key
    assert singleton_instance1.transient == singleton_instance1_loaded.transient


if __name__ == "__main__":
    # singleton implementation works fine as long as metaclass = SingletonMeta
    test_singleton()

    # pickling instances works fine if metaclass != SingletonMeta
    # but raises `TypeError: cannot pickle '_thread.RLock' object` if metaclass = SingletonMeta
    test_singleton_pickle()

    # raises "TypeError: cannot pickle '_thread.RLock' object"
    cloudpickle.dumps(test_singleton)
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