如何创建正确垃圾收集的自定义生成器类

Question

我正在尝试在 Python 中编写一个充当生成器对象的类，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。

这里有一些解释性代码： 如果你中断了一个生成器，那么当它被垃圾回收时，Python 会在生成器对象上调用.close()，这会向生成器抛出一个GeneratorExit 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，如果您尝试通过从 collections.abc.Generator 继承来实现自己的生成器对象，Python 似乎没有注意到它应该在收集对象时调用 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

我认为继承 Generator 足以让 python 相信我的 CustomGeneratorClass 是一个生成器，并且应该在垃圾收集时调用 .close()。

我认为这与“生成器对象”是某种特殊的 Generator 的事实有关：

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GeneratorType吗？

关于 python 如何决定调用.close() 有什么我不明白的地方吗？

如何确保在我的自定义生成器上调用 .close()？

---

此问题与How to write a generator class 不重复。 对于实际创建一个生成器类，该问题的公认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。

Answer 1

PEP342，声明：

[generator].__del__() 是[generator].close() 的包装器。这将在生成器对象被垃圾回收时调用...

collections.abc 中的 Generator 类没有实现 __del__，它的超类或元类也没有。

将__del__ 的这个实现添加到问题中的类会导致锁被释放：

class CustomGeneratorClass(Generator):

    ...

    def __del__(self):
        self.close() 

输出：

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

警告：

我对 Python 中对象终结的复杂性没有经验，因此应该严格检查这个建议，并测试到破坏。尤其要考虑language reference中关于__del__的警告。

---

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住这样做。