[New-bugs-announce] [issue27972] Confusing error during cyclic yield

[Max von Tettenborn]

New submission from Max von Tettenborn:

Below code reproduces the problem. The resulting error is a RecursionError and it is very hard to trace that to the cause of the problem, which is the runner task and the stop task yielding from each other, forming a deadlock.

I think, an easy to make mistake like that should raise a clearer exception. And maybe I am mistaken, but it should in principle be possible for the event loop to detect a cyclic yield, right?


import asyncio


class A:
    @asyncio.coroutine
    def start(self):
        self.runner_task = asyncio.ensure_future(self.runner())

    @asyncio.coroutine
    def stop(self):
        self.runner_task.cancel()
        yield from self.runner_task

    @asyncio.coroutine
    def runner(self):
        try:
            while True:
                yield from asyncio.sleep(5)
        except asyncio.CancelledError:
            yield from self.stop()
            return


def do_test():
    @asyncio.coroutine
    def f():
        a = A()
        yield from a.start()
        yield from asyncio.sleep(1)
        yield from a.stop()

    asyncio.get_event_loop().run_until_complete(f())

----------
components: asyncio
messages: 274547
nosy: Max von Tettenborn, gvanrossum, haypo, yselivanov
priority: normal
severity: normal
status: open
title: Confusing error during cyclic yield
type: behavior
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27972>
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