Hi Chris,

This specific test is easy to write (mock first to return a resolved future, 2nd to block and 3rd to assert False)

OTOH complexity of the general case is unbounded and generally exponential. It's akin to testing multithreaded code.

(There's an academic publication from Microsoft where they built a runtime that would run each test really many times, where scheduler is rigged to order runnable tasks differently on each run. I hope someone rewrites this for asyncio)

Certainty [better] tools are needed, and ultimately it's a tradeoff between sane/understable/maintainable tests and testing deeper/more corner cases.

Just my 2c...

On Jul 1, 2017 12:11, "Chris Jerdonek" <chris.jerdonek@gmail.com> wrote:

I have a question about testing async code.

Say I have a coroutine:

async def do_things():
await do_something()
await do_more()
await do_even_more()

And future:

task = ensure_future(do_things())

Is there a way to write a test case to check that task.cancel() would
behave correctly if, say, do_things() is waiting at the line
do_more()?

In real life, this situation can happen if a function like the
following is called, and an exception happens in one of the given
tasks. One of the tasks in the "pending" list could be at the line
do_more().

done, pending = await asyncio.wait(tasks,
return_when=asyncio.FIRST_EXCEPTION)

But in a testing situation, you don't necessarily have control over
where each task ends up when FIRST_EXCEPTION occurs.

Thanks,
--Chris
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