as_completed identifying coroutie objects - python-asyncio

Question

I'm using asyncio to await set of coroutines in following way:
# let's assume we have fn defined and that it can throw an exception
coros_objects = []
for x in range(10):
coros_objects.append(fn(x))
for c in asyncio.as_completed(coros_objects):
try:
y = await c
exception:
# something
# if possible print(x)
Question is how can I know which coroutine failed and for which argument?
I could append "x" to the outputs but this would give me info about successful executions only.
I can know that form order because it's different from the order of coros_objects
Can I somehow identify what coro just yielded result?

Answer 1

Question is how can I know which coroutine failed and for which argument?
You can't with the current as_completed. Once this PR is merged, it will be possible by attaching the information to the future (because as_completed will then yield the original futures). At the moment there are two workarounds:
wrap the coroutine execution in a wrapper that catches exceptions and stores them, and also stores the original arguments that you need, or
not use as_completed at all, but write your own loop using tools like asyncio.wait.
The second option is easier than most people expect, so here it is (untested):
# create a list of tasks and attach the needed information to each
tasks = []
for x in range(10):
t = asyncio.create_task(fn(x))
t.my_task_arg = x
tasks.append(t)
# emulate as_completed with asyncio.wait()
while tasks:
done, tasks = await asyncio.wait(tasks, return_when=asyncio.FIRST_COMPLETED)
for t in done:
try:
y = await t
except Exception as e:
print(f'{e} happened while processing {t.my_task_arg}')