如何创建正确垃圾收集的自定义生成器类

我正在尝试用 Python 编写一个类，其行为类似于生成器对象，特别是当它被垃圾收集时.close()被调用。这很重要，因为这意味着当生成器被中断时，我可以确保它会自行清理，例如关闭文件或释放锁。

这是一些解释性代码： 如果你中断一个生成器，那么当它被垃圾收集时，Python 会调用.close()在生成器对象上，它会抛出一个GeneratorExit生成器中出现错误，可以捕获该错误以进行清理，如下所示：

from threading import Lock

lock = Lock()

def CustomGenerator(n, lock):
    lock.acquire()
    print("Generator Started: I grabbed a lock")
    try:
        for i in range(n):
            yield i
    except GeneratorExit:
        lock.release()
        print("Generator exited early: I let go of the lock")
        raise
    print("Generator finished successfully: I let go of the lock")

for i in CustomGenerator(100, lock):
    print("Received ", i)
    time.sleep(0.02)
    if i==3:
        break

if not lock.acquire(blocking=False):
    print("Oops: Finished, but lock wasn't released")
else:
    print("Finished: Lock was free")
    lock.release()

Generator Started: I grabbed a lock
Received  0
Received  1
Received  2
Received  3
Generator exited early: I let go of the lock
Finished: Lock was free

但是，如果您尝试通过继承来实现自己的生成器对象collections.abc.Generator，Python似乎没有注意到它应该在对象被收集时调用close：

from collections.abc import Generator
class CustomGeneratorClass(Generator):
    def __init__(self, n, lock):
        super().__init__()
        self.lock = lock
        self.lock.acquire()
        print("Generator Class Initialised: I grabbed a lock")
        self.n = n
        self.c = 0

    def send(self, arg):
        value = self.c
        if value >= self.n:
            raise StopIteration
        self.c += 1
        return value

    def throw(self, type, value=None, traceback=None):
        print("Exception Thrown in Generator: I let go of the lock")
        self.lock.release()
        raise StopIteration

for i in CustomGeneratorClass(100, lock):
    print("Received ", i)
    time.sleep(0.02)
    if i==3:
        break

if not lock.acquire(blocking=False):
    print("Oops: Finished, but lock wasn't released")
else:
    print("Finished: Lock was free")
    lock.release()

Generator Class Initialised: I grabbed a lock
Received  0
Received  1
Received  2
Received  3
Oops: Finished, but lock wasn't released

我以为继承Generator足以让 python 相信我的 CustomGeneratorClass 是一个生成器并且应该具有.close()垃圾收集时调用它。

我认为这与“生成器对象”是某种特殊的事实有关Generator:

from types import GeneratorType

c_gen = CustomGenerator(100)
c_gen_class = CustomGeneratorClass(100)

print("CustomGenerator is a Generator:", isinstance(c_gen, Generator))
print("CustomGenerator is a GeneratorType:",isinstance(c_gen, GeneratorType))

print("CustomGeneratorClass is a Generator:",isinstance(c_gen_class, Generator))
print("CustomGeneratorClass is a GeneratorType:",isinstance(c_gen_class, GeneratorType))

CustomGenerator is a Generator: True
CustomGenerator is a GeneratorType: True
CustomGeneratorClass is a Generator: True
CustomGeneratorClass is a GeneratorType: False

我可以创建一个用户定义的类对象吗GeneratorType?

关于 python 如何决定调用什么，有什么我不明白的地方吗.close() on?

我怎样才能确保.close()在我的自定义生成器上调用？

这个问题不是重复的如何编写生成器类。 对于实际创建一个生成器类，该问题的可接受答案确实推荐了我在这里尝试的结构，它是一个生成器类，但没有正确地进行垃圾收集，如上面的代码所示。

PEP342， 状态：

[generator].__del__()是一个包装器[generator].close()。当生成器对象被垃圾收集时，这将被调用......

中的生成器类集合.abc不实施__del__，它的超类或元类也没有。

添加这个实现__del__到问题中的类会导致锁被释放：

class CustomGeneratorClass(Generator):

    ...

    def __del__(self):
        self.close() 

Output:

Generator Class Initialised: I grabbed a lock
Recieved  0
Recieved  1
Recieved  2
Recieved  3
Exception Thrown in Generator: I let go of the lock
Finished: Lock was free

Caveat:

我对 Python 中对象终结的复杂性没有经验，因此应该仔细检查此建议，并进行破坏测试。特别是关于以下警告__del__ in the 语言参考应该被考虑。

更高级别的解决方案是在上下文管理器中运行生成器

with contextlib.closing(CustomGeneratorClass(100, lock)):
    # do stuff

但这很麻烦，并且依赖于代码的用户记得这样做。