In developer mode (-X dev), ResourceWarning is only emited once per line numbers

[vstinner]

BPO	32089
Nosy	@ncoghlan, @pitrou, @vstinner, @serhiy-storchaka
PRs	bpo-32089: Fix warnings filters in dev mode #4482 bpo-32089: Use default action for ResourceWarning #4584

^{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 2017-11-27.11:14:41.427>
created_at = <Date 2017-11-20.14:57:20.398>
labels = ['3.7']
title = 'In developer mode (-X dev), ResourceWarning is only emited once per line numbers'
updated_at = <Date 2017-11-27.11:14:41.426>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-11-27.11:14:41.426>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-11-27.11:14:41.427>
closer = 'vstinner'
components = []
creation = <Date 2017-11-20.14:57:20.398>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 32089
keywords = ['patch']
message_count = 14.0
messages = ['306548', '306603', '306609', '306635', '306637', '306638', '306639', '306641', '306646', '306868', '306905', '306946', '307049', '307050']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'pitrou', 'vstinner', 'serhiy.storchaka']
pr_nums = ['4482', '4584']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue32089'
versions = ['Python 3.7']

[vstinner]

The -X dev mode currently *hides* some ResourceWarning warnings:

$ cat x.py 
def func():
    open('/etc/issue')

func()
func()

$ ./python x.py 
x.py:2: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
  open('/etc/issue')
x.py:2: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
  open('/etc/issue')

haypo@selma$ ./python -X dev x.py
x.py:2: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
open('/etc/issue')

The problem is that the "-W default" inserted by -X dev overrides the final filter on ResourceWarning:

$ ./python -X dev -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('default',
  re.compile('', re.IGNORECASE),
  <class 'Warning'>,
  re.compile(''),
  0),
 ('ignore', None, <class 'BytesWarning'>, None, 0),
 ('always', None, <class 'ResourceWarning'>, None, 0)]

[vstinner]

Attached PR 4482 fixes warnings filters:

haypo@selma$ ./python -X dev -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('ignore', None, <class 'BytesWarning'>, None, 0),
('always', None, <class 'ResourceWarning'>, None, 0),
('default', None, <class 'Warning'>, None, 0)]

[vstinner]

New changeset 09f3a8a by Victor Stinner in branch 'master':
bpo-32089: Fix warnings filters in dev mode (bpo-4482)
09f3a8a

[pitrou]

I don't agree with this. You're comparing "-X dev" with a pydebug build of Python, not with a normal Python.

Your example shows the problem. If a program calls func() 1000 times in a row, they will show the same warning 1000 times. What does it bring to have the exact same warning (and line number) displayed 1000 times instead of once? Nothing. There is a reason the filter "default" isn't the same as "always", and it's that "always" can be completely overwhelming.

[vstinner]

If a program calls func() 1000 times in a row, they will show the same warning 1000 times. What does it bring to have the exact same warning (and line number) displayed 1000 times instead of once? Nothing. There is a reason the filter "default" isn't the same as "always", and it's that "always" can be completely overwhelming.

I don't know the rationale of the "always" action rather than "default". The choice was made 7 years ago, when Georg Brandl implemented the new ResourceWarning category: commit 08be72d.

The problem of ResourceWarning is that the warning logs where the last reference to the resource was cleared, or where the a garbage collection was triggered. Multiple resources can be created from different places, but all "die" at the same place.

In Python 3.6, I enhanced ResourceWarning to log also the traceback where the leaked resource has been created, if the tracemalloc is tracing memory allocations. When tracemalloc is used, it's useful to log ResourceWarning multiple times even if the warning itself is logged multipletimes. Well, that's an unusual usage of the ResourceWarning, we can also suggest to use -W always::ResourceWarning in that case.

I don't agree with this. You're comparing "-X dev" with a pydebug build of Python, not with a normal Python.

In general, I would like to offer the same experience in "development mode" (-X dev) and with a pydebug build.

For ResourceWarning, your rationale only concerns pydebug build, no?

[pitrou]

Le 21/11/2017 à 14:46, STINNER Victor a écrit :

I don't know the rationale of the "always" action rather than "default".

Neither do I. But I don't think it matters a lot. pydebug builds are
almost only used by Python core developers. "developer mode" is target
at every Python developer.

The problem of ResourceWarning is that the warning logs where the last reference to the resource was cleared, or where the a garbage collection was triggered. Multiple resources can be created from different places, but all "die" at the same place.

If they have different names, they will be logged separately.

>>> def f(fname):
...     open(fname)
...
>>> f('setup.py')
__main__:2: ResourceWarning: unclosed file <_io.TextIOWrapper
name='setup.py' mode='r' encoding='UTF-8'>
>>> f('setup.py')
>>> f('LICENSE.txt')
__main__:2: ResourceWarning: unclosed file <_io.TextIOWrapper
name='LICENSE.txt' mode='r' encoding='UTF-8'>
>>> f('LICENSE.txt')
>>>

For ResourceWarning, your rationale only concerns pydebug build, no?

Why? I'm talking about "-X dev", not pydebug builds.

[pitrou]

I forgot: the snippet above was made with "python -Wdefault::ResourceWarning".

[serhiy-storchaka]

See also bpo-27535.

[vstinner]

Antoine:

If they have different names, they will be logged separately.

Oh wow, nice behaviour, I like it :-)

Antoine:

> For ResourceWarning, your rationale only concerns pydebug build, no?
Why? I'm talking about "-X dev", not pydebug builds.

Oops sorry, typo (brain error), I mean: "also concerns".

Serhiy:

See also bpo-27535.

Oh, not leaking memory in warnings registies is a nice side effect of the "always" action :-)

--

It's much more complicated than what I expected to get the "correct behaviour" for warnings :-/

[vstinner]

Antoine: What do you think of using the "default" action for pydebug build and -X dev?

[pitrou]

Antoine: What do you think of using the "default" action for pydebug build and -X dev?

That would be fine with me.

[ncoghlan]

+1 from me for using "default" instead of "always" for ResourceWarning.

Folks can always combine "-X tracemalloc" with "-W always::ResourceWarning" if want to ensure they see absolutely every resource warning, rather than only representative ones.