asyncio.sleep() does not adhere to time.sleep() behavior for negative numbers

[Marco-Sulla]

BPO	39698
Nosy	@asvetlov, @1st1, @Marco-Sulla

^{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 2020-02-20.19:17:33.548>
created_at = <Date 2020-02-20.11:44:23.841>
labels = ['type-bug', '3.9', 'expert-asyncio']
title = 'asyncio.sleep() does not adhere to time.sleep() behavior for negative numbers'
updated_at = <Date 2020-02-26.19:46:37.563>
user = 'https://github.com/Marco-Sulla'

bugs.python.org fields:

activity = <Date 2020-02-26.19:46:37.563>
actor = 'Marco Sulla'
assignee = 'none'
closed = True
closed_date = <Date 2020-02-20.19:17:33.548>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2020-02-20.11:44:23.841>
creator = 'Marco Sulla'
dependencies = []
files = []
hgrepos = []
issue_num = 39698
keywords = []
message_count = 7.0
messages = ['362314', '362321', '362345', '362349', '362519', '362673', '362724']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'Marco Sulla']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39698'
versions = ['Python 3.9']

[Marco-Sulla]

Python 3.9.0a3+ (heads/master-dirty:f2ee21d858, Feb 19 2020, 23:19:22) 
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import time
>>> time.sleep(-1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: sleep length must be non-negative
>>> import asyncio
>>> async def f():
...     await asyncio.sleep(-1)
...     print("no exception")
... 
>>> asyncio.run(f())
no exception

I think that also asyncio.sleep() should raise ValueError if the argument is less than zero.

[asvetlov]

The ship has sailed, this change breaks a lot of existing code without a strong reason.
I recall very many cases in third-party libraries and commercial applications where a negative argument for asyncio.sleep() is processed as asyncio.sleep(0) without failure.

[1st1]

The ship has sailed, this change breaks a lot of existing code without a strong reason.

Yes.

It's a common thing to compute asyncio.sleep delay and sometimes it goes negative. The current behavior is part of our API now.

[Marco-Sulla]

I recall very many cases in third-party libraries and commercial applications

Source?

[Marco-Sulla]

I see that many breaking changes was done in recent releases. I get only the ones for asyncio in Python 3.8:

https://bugs.python.org/issue36921
https://bugs.python.org/issue36373
https://bugs.python.org/issue34790
https://bugs.python.org/issue32528
https://bugs.python.org/issue34687
https://bugs.python.org/issue32314

So I suppose the ship isn't sailed yet.

Passing a negative number to a function that should sleep the task for x seconds is a mistake. And mistakes should never pass silently.

Furthermore, coherence matters. It's really confusing that two functions in two builtin modules that are quite identical have a different behavior.

IMHO, deprecating and then removing support for negative argument in asyncio.sleep() is very much less breaking compared to issues bpo-36921 and bpo-36373 .