using asyncio.iscoroutinefunction() on a functools.partial object

[mathieui]

BPO	23519
Nosy	@gvanrossum, @vstinner, @mathieui, @1st1
Files	example_partial.py

^{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 2015-02-26.17:52:33.200>
created_at = <Date 2015-02-25.11:00:39.941>
labels = ['type-feature', 'invalid', 'expert-asyncio']
title = 'using asyncio.iscoroutinefunction() on a functools.partial object'
updated_at = <Date 2017-06-09.02:28:01.054>
user = 'https://github.com/mathieui'

bugs.python.org fields:

activity = <Date 2017-06-09.02:28:01.054>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2015-02-26.17:52:33.200>
closer = 'gvanrossum'
components = ['asyncio']
creation = <Date 2015-02-25.11:00:39.941>
creator = 'mathieui'
dependencies = []
files = ['38246']
hgrepos = []
issue_num = 23519
keywords = []
message_count = 8.0
messages = ['236569', '236570', '236592', '236660', '236668', '236693', '295475', '295478']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'vstinner', 'mathieui', 'yselivanov', 'curtmcd']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23519'
versions = ['Python 3.4', 'Python 3.5']

[mathieui]

Using iscoroutinefunction() on an object returned by functools.partial() should return True if the function wrapped was a coroutine function.

(a recursive loop like the one in asyncio/events.py get_function_source() may be what needs to be done)

[vstinner]

Take a look at asyncio.events._get_function_source() which supports wrapped functions and functools.partial.

Can you propose a patch?

If iscoroutinefunction() becomes "expensive", we might make checks optional, only run them in debug mode.

[gvanrossum]

I recall discussing this before (maybe on the tulip list). I am firmly against. It is a slippery slope -- why inspect a partial but not a lambda? Plus there is no use case.

[vstinner]

Plus there is no use case.

Mathieu: can you maybe give some examples? How are you using functools.partial with coroutines?

[mathieui]

Using functools.partial with coroutines would be mostly out of convenience, in order to avoid having factories in that return parametrized coroutine functions. I guess in such cases it might be better to create a two-lines wrapper around partial() to make it return a coroutine rather than change the stdlib for that.

In the attached file is an example of such use, where EventNotifier is a Protocol which receives external events and triggers event handlers based on that, and where the add_event_handler function checks if the handler is a coroutine function. In which case it uses asyncio.async to schedule the handler call; otherwise it uses loop.call_soon.

You can close this, I guess.

[gvanrossum]

Yeah, your "add event handler" routine shouldn't be so picky to insist that iscoroutinefunction() returns True. It should just call the thing and verify that it has returned a coroutine object (asyncio.iscoroutine()).

[curtmcd]

There are use cases for this. I hit the problem and eventually wound up here, so I'd like to make a case to re-open.

My project has a callback registry of asyncio handler routines.

register_callback(condition1, handler1)
register_callback(condition2, handler2)
...

I want to register many callbacks, but use only one handler and an argument to differentiate it. I realize our callback systems should provide for a cookie, but it doesn't.

register_callback(condition1, functools.partial(handler, 'detail1'))
register_callback(condition2, functools.partial(handler, 'detail2'))

The callback registry makes sure iscoroutinefunction(handler) because we don't want to defer error checking to the distant future. But iscoroutinefunction() returns False for the partial function. I was hopeful that this might work, but alas, no:

register_callback(condition1,
                  asyncio.coroutine(functools.partial(handler, 'detail1')))

[gvanrossum]

Use a lambda instead of partial. It's more pythonic.

[graingert]

this is fixed in #79071