asyncio.run interacts surprisingly with debug mode

[hauntsaninja]

BPO	41696
Nosy	@asvetlov, @1st1, @miss-islington, @hauntsaninja
PRs	bpo-41696: Fix handling of debug mode in asyncio.run #22069 [3.9] bpo-41696: Fix handling of debug mode in asyncio.run (GH-22069) #22071 [3.8] bpo-41696: Fix handling of debug mode in asyncio.run (GH-22069) #22072

^{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 2022-03-12.20:01:32.982>
created_at = <Date 2020-09-03.00:33:50.817>
labels = ['expert-asyncio']
title = 'asyncio.run interacts surprisingly with debug mode'
updated_at = <Date 2022-03-12.20:01:32.982>
user = 'https://github.com/hauntsaninja'

bugs.python.org fields:

activity = <Date 2022-03-12.20:01:32.982>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-12.20:01:32.982>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2020-09-03.00:33:50.817>
creator = 'hauntsaninja'
dependencies = []
files = []
hgrepos = []
issue_num = 41696
keywords = ['patch']
message_count = 4.0
messages = ['376268', '376269', '376282', '376318', '376319']
nosy_count = 4.0
nosy_names = ['asvetlov', 'yselivanov', 'miss-islington', 'hauntsaninja']
pr_nums = ['22069', '22071', '22072']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41696'
versions = []

[hauntsaninja]

To quote the docs (https://docs.python.org/3/library/asyncio-dev.html):

By default asyncio runs in production mode. In order to ease the development asyncio has a debug mode.

There are several ways to enable asyncio debug mode:

    Setting the PYTHONASYNCIODEBUG environment variable to 1.

    Using the -X dev Python command line option.

    Passing debug=True to asyncio.run().

    Calling loop.set_debug().

Unfortunately, this documentation is misleading (or wrong) when using asyncio.run. For instance, from a naive read of the above, I'd expect debug mode to be True for many of the following:

~/tmp λ cat test57.py
import asyncio
import time

async def foo():
    loop = asyncio.get_event_loop()
    time.sleep(2)
    print(loop.get_debug())

print(asyncio.coroutines._DEBUG)
asyncio.run(foo())
~/tmp λ python3 --version       
Python 3.7.3

# expected
~/tmp λ python3 test57.py
False
False

# surprising, would expect this to work
~/tmp λ python3 -X dev test57.py
True
False

# surprising, would expect this to work
~/tmp λ PYTHONASYNCIODEBUG=1 python3 test57.py
True
False

# expected
~/tmp λ vim test57.py
~/tmp λ cat test57.py
import asyncio
import time

async def foo():
    loop = asyncio.get_event_loop()
    time.sleep(2)
    print(loop.get_debug())

print(asyncio.coroutines._DEBUG)
asyncio.run(foo(), debug=True)
~/tmp λ python3 test57.py       
False
True
Executing <Task finished coro=<foo() done, defined at test57.py:4> result=None created at /Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py:563> took 2.003 seconds
~/tmp λ 

This appears to be because asyncio.run sets debug mode here:

cpython/Lib/asyncio/runners.py

Line 42 in 4a97b15

loop.set_debug(debug)

We should either change asyncio.run to use the existing value of debug mode if left unspecified, or we should update the documentation to explicitly call out that the environment variable and command line flag will not work when using asyncio.run

[1st1]

Yes, this is a bug. The default for the debug parameter should be None and it should only call loop.set_debug if a non-None bool value was explicitly passed. Care to submit a PR?

[1st1]

New changeset 0770ad9 by Shantanu in branch 'master':
bpo-41696: Fix handling of debug mode in asyncio.run (bpo-22069)
0770ad9

[1st1]

New changeset 1f5f127 by Miss Islington (bot) in branch '3.8':
bpo-41696: Fix handling of debug mode in asyncio.run (GH-22069) (bpo-22072)
1f5f127

[1st1]

New changeset a268005 by Miss Islington (bot) in branch '3.9':
bpo-41696: Fix handling of debug mode in asyncio.run (GH-22069) (bpo-22071)
a268005