Python生成器中的GeneratorExit [英] GeneratorExit in Python generator

问题描述

我写了一个关于Python生成器的测试程序.但是我得到了一个意料之外的错误.而且我不知道该怎么解释.让我向您展示代码:

I wrote a test program about Python generator. But I got an error that is not expected. And I don't know how to explain it. Let me show you the code:

def countdown(n):
    logging.debug("Counting down")
    while n > 0:
        try:
            yield n
        except GeneratorExit:
            logging.error("GeneratorExit")
        n -= 1


if __name__ == '__main__':
    c = countdown(10)
    logging.debug("value: %d", c.next())

我认为它应该可以正常运行.但是输出是:

I thought it should run without any problem. But the output is:

# ./test.py
[2015/06/16 04:10:49] DEBUG    - Counting down     
[2015/06/16 04:10:49] DEBUG    - value: 10 
[2015/06/16 04:10:49] ERROR    - GeneratorExit
Exception RuntimeError: 'generator ignored GeneratorExit' in <generator object countdown at 0x7f9934407640> ignored

为什么最后一行出现错误.我不知道为什么触发了GeneratorExit异常.我错过了发电机吗?我也将代码输入到交互式python shell中，一切正常.这怎么会发生?

Why is there an error at the last line. I don't know why I triggered the GeneratorExit exception. Is there something aobut generator I missed? I also typed the code into interactive python shell, and everything is OK. How can this happen?

推荐答案

假设您具有以下生成器:

Suppose you have the following generator:

def gen():
    with open('important_file') as f:
        for line in f:
            yield line

然后您next将其丢弃一次:

g = gen()
next(g)
del g

生成器的控制流从不离开with块，因此不会关闭文件.为了防止这种情况，当生成器被垃圾回收时，Python调用其close方法，该方法在生成器最后一次yield的生成点引发GeneratorExit异常.此异常旨在触发所有没有机会运行的finally块或上下文管理器__exit__.

The generator's control flow never leaves the with block, so the file doesn't get closed. To prevent this, when a generator is garbage-collected, Python calls its close method, which raises a GeneratorExit exception at the point from which the generator last yielded. This exception is intended to trigger any finally blocks or context manager __exit__s that didn't get a chance to run.

当您抓住GeneratorExit并继续前进时，Python会看到生成器未正确退出.由于这可能表明资源未正确释放，因此Python将其报告为RuntimeError.

When you catch the GeneratorExit and keep going, Python sees that the generator didn't exit properly. Since that can indicate that resources weren't properly released, Python reports this as a RuntimeError.

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