在协程中尝试/除外/最终

如何解决在协程中尝试/除外/最终

class DemoException(Exception):   
    """An exception type for the demonstration."""

def demo_exc_handling():
    print('-> coroutine started')
    while True:
        try:
            x = yield
        except DemoException:  # <1>
            print('*** DemoException handled. Continuing...')
        else:  # <2>
            print('-> coroutine received: {!r}'.format(x))
        finally:
            print('-> 1111111111coroutine ending')
    raise RuntimeError('This line should never run.')  

if __name__ == '__main__':
exc_coro = demo_exc_handling()
next(exc_coro)
exc_coro.send(11)

我得到以下输出：

-> coroutine started
-> coroutine received: 11
-> 1111111111coroutine ending
-> 1111111111coroutine ending

我想知道为什么finally语句执行两次？ 我将非常感谢您的帮助。

解决方法

这与协程或异常无关。只需生成器即可重现：

def demo_exc_handling():
    print('-> coroutine started')
    while True:
        try:
            x = yield
        finally:
            print('-> 1111111111coroutine ending')
    raise RuntimeError('This line should never run.')

if __name__ == '__main__':
    exc_coro = demo_exc_handling()
    next(exc_coro)
    next(exc_coro)

以下是输出：

-> coroutine started
-> 1111111111coroutine ending
-> 1111111111coroutine ending

您可能想做的是在try-finally内部进行while循环：

def demo_exc_handling():
    print('-> coroutine started')
    try:
        while True:
            x = yield
    finally:
        print('-> 1111111111coroutine ending')
    raise RuntimeError('This line should never run.')

而是输出

-> coroutine started
-> 1111111111coroutine ending

,

它打印两次，因为即使主程序结束，finally子句也会执行。

协程在主程序结束时第二次产生

让我们产生迭代次数并添加一些打印输出以查看它

class DemoException(Exception):   
    """An exception type for the demonstration."""
    pass

def demo_exc_handling():
    print('-> coroutine started')
    i = 1
    while True:
        try:
            print(f"iteration {i}")
            x = yield i
        except DemoException:  # <1>
            print('*** DemoException handled. Continuing...')
        else:  # <2>
            print('-> coroutine received: {!r}'.format(x))
        finally:
            print('-> 1111111111coroutine ending')
        i += 1
    raise RuntimeError('This line should never run.')  

if __name__ == '__main__':
    exc_coro = demo_exc_handling()
    print("initialised")
    y = next(exc_coro)
    print(f"got {y}")
    exc_coro.send(11)

产生

initialised
-> coroutine started
iteration 1
got 1
-> coroutine received: 11
-> 1111111111coroutine ending
iteration 2
-> 1111111111coroutine ending