Allow to provide custom exception handler to asyncio.run()

[tomaszdrozdz]

BPO	41202
Nosy	@asvetlov, @1st1, @aeros, @tomaszdrozdz
PRs	bpo-41202: Allow to provide custom exception handler to asyncio.run() #21295

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-07-11.03:08:49.273>
created_at = <Date 2020-07-03.14:26:06.886>
labels = ['type-feature', '3.8', 'expert-asyncio']
title = 'Allow to provide custom exception handler to asyncio.run()'
updated_at = <Date 2020-07-11.03:08:49.272>
user = 'https://github.com/tomaszdrozdz'

bugs.python.org fields:

activity = <Date 2020-07-11.03:08:49.272>
actor = 'aeros'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-11.03:08:49.273>
closer = 'aeros'
components = ['asyncio']
creation = <Date 2020-07-03.14:26:06.886>
creator = 'tomaszdrozdz'
dependencies = []
files = []
hgrepos = []
issue_num = 41202
keywords = ['patch']
message_count = 9.0
messages = ['372934', '373181', '373226', '373227', '373348', '373390', '373408', '373463', '373508']
nosy_count = 4.0
nosy_names = ['asvetlov', 'yselivanov', 'aeros', 'tomaszdrozdz']
pr_nums = ['21295']
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41202'
versions = ['Python 3.8']

[tomaszdrozdz]

I wish we had:

asyncio.run(coro, *, debug=False, excepton_handler=None)

so we could provide custome exception handler function for the loop.

[1st1]

The idiomatic way:

async def main():
  loop = asyncio.get_running_loop()
  loop.set_exception_handler(...)
  # other code

asyncio.run(main())

We don't want to add new arguments to asyncio.run as there would be too many.

[tomaszdrozdz]

https://docs.python.org/3/library/asyncio-eventloop.html#error-handling-api

Here we can read:

Application developers should typically use the high-level asyncio functions, such as asyncio.run(), and should rarely need to reference the loop object or call its methods. This section is intended mostly for authors of lower-level code, libraries, and frameworks, who need finer control over the event loop behavior.

So as I understand this - I should not use
asyncio.get_running_loop
loop.set_exception_handler(...)

Or maybe event loop should not be in "Low level api" ???

[asvetlov]

I agree with Yuri.

Usually, you don't need overriding of the default exception handler.
Indeed, if you really need this low-level API I see nothing wrong with asyncio.get_running_loop() call.

[aeros]

Yep, having to set a custom exception handler definitely constitutes as needing "finer control over the event loop behavior". There's absolute nothing wrong with using the low-level API when you need further customization, but we try to minimize the high-level API as much as possible to make it simple for the average user. I suspect that the majority of asyncio users don't have a need to customize the exception handler beyond the default settings.

[tomaszdrozdz]

OK.
I understand.

So how about maybe:

    def run(main, *, debug=False, loop=None):  
        ...  
    
        if loop:  
            loop = events.new_event_loop()

So we could customize loop like:

    loop = events.new_event_loop()  
    loop.set_XXX(...)  
    asyncio.run(my_coro, loop=loop)  

Just what tehn with debug parameter ?

[1st1]

So how about maybe:

That wouldn't work. You still haven't explained what's wrong with calling loop = asyncio.get_running_loop() inside async def main(). That literally solves all problems without the need of us modifying any APIs.

[tomaszdrozdz]

I just wanted to call

    def main():
        asyncio.run(...)

But I can go with Your aproach.
Thanks for discusion.

Should I set status for this issue for closed with resolution rejected ?

Should I delete branch on my forked git repo ?
Can I delete my forked git repo ?

[aeros]

Should I set status for this issue for closed with resolution rejected ?

I'll proceed with closing the issue.

Should I delete branch on my forked git repo ?
Can I delete my forked git repo ?

Might as well delete the branch, but the forked repo might be useful to keep around should you choose to open a PR to CPython in the future (although you can also just create a new one later). If you are interested in working on something else in the future, I'd recommend looking at the "newcomer friendly"/"easy" issues, or just looking for an existing open issue that you're interested in helping with. In most cases, working with a clearly defined issue is much easier than trying to propose a new one and getting it merged.