try/except/finally in Coroutine

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)

I get the following output:

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

I want to know why the finally statement executes twice? I would be very grateful for any help.

Answer 1

It prints twice because the finally clause executes even if the main program ends.

The coroutine is yielding the second time when the main program ends

Let's yield the iteration number and add a couple of printouts to see it

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)

produces

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

Answer 2

This has nothing to do with coroutines or exceptions. Just a generator will reproduce fine:

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)

Here is the output:

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

What you probably meant to do was have the while loop INSIDE the try-finally:

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

which instead outputs

-> coroutine started
-> 1111111111coroutine ending