better exception message when an unsupported object is passed to async for (pep 492)

[1st1]

BPO	24995
Nosy	@ncoghlan, @vstinner, @1st1

^{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 = 'https://github.com/1st1'
closed_at = None
created_at = <Date 2015-09-03.21:13:47.658>
labels = ['interpreter-core', 'type-feature', '3.9']
title = 'better exception message when an unsupported object is passed to `async for` (pep 492)'
updated_at = <Date 2019-11-22.09:33:49.517>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2019-11-22.09:33:49.517>
actor = 'gvanrossum'
assignee = 'yselivanov'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2015-09-03.21:13:47.658>
creator = 'yselivanov'
dependencies = []
files = []
hgrepos = []
issue_num = 24995
keywords = []
message_count = 6.0
messages = ['249689', '249691', '249693', '249696', '249887', '357260']
nosy_count = 3.0
nosy_names = ['ncoghlan', 'vstinner', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue24995'
versions = ['Python 3.9']

[1st1]

Should we raise something like "'int' object is not an asynchronous iterable", instead of "'async for' requires an object with __aiter__ method, got int"?

>>> foo().send(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in foo
TypeError: 'async for' requires an object with __aiter__ method, got int


>>> for i in 1: pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

[gvanrossum]

Did you get multiple complaints about this? The existing error doesn't seem so bad.

[vstinner]

IMHO the current message is clear enough.

[1st1]

I saw this reddit thread: https://www.reddit.com/r/Python/comments/3ewnwq/async_function_as_generator_for_async_for/ where people are struggling with understanding how 'async for' works. Which led me to check what error messages we raise when we pass a wrong object to it. Turns out the message is fine, but different from what we raise on iter(1) etc.

[ncoghlan]

In tandem with an "asynchronous iterable" glossary entry, that could be a nice improvement.

Even without such a change the first hit for "python asynchronous iterable" is PEP-492, so it will get folks pointed in the right direction.

[gvanrossum]

Do what you think is best.

[kumaraditya303]

FWIW the existing message is clear enough and this seems abandoned for over 7 years. I'll close this unless anyone objects otherwise.

[gvanrossum]

Just close it.