在协程中尝试/除外/最终

Question

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finally语句为什么会执行两次？ 如果有任何帮助，我将不胜感激。

Answer 1

它会打印两次，因为即使主程序结束，finally 子句也会执行。

主程序结束时协程第二次让步

让我们生成迭代次数并添加几个打印输出来查看它

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)

生产

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

Answer 2

这与协程或异常无关。只需一个生成器就可以很好地重现：

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)

这是输出：

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

您可能打算在 try-finally 中使用 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.')

改为输出

-> coroutine started
-> 1111111111coroutine ending