_warnings.warn_explicit() should try to import the warnings module

[vstinner]

BPO	26592
Nosy	@vstinner
Files	warnings_import.patch

^{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 2016-03-22.23:56:19.424>
created_at = <Date 2016-03-19.13:34:14.782>
labels = ['type-feature']
title = '_warnings.warn_explicit() should try to import the warnings module'
updated_at = <Date 2016-03-22.23:56:19.423>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2016-03-22.23:56:19.423>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2016-03-22.23:56:19.424>
closer = 'vstinner'
components = []
creation = <Date 2016-03-19.13:34:14.782>
creator = 'vstinner'
dependencies = []
files = ['42217']
hgrepos = []
issue_num = 26592
keywords = ['patch']
message_count = 2.0
messages = ['262049', '262219']
nosy_count = 2.0
nosy_names = ['vstinner', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26592'
versions = ['Python 3.6']

[vstinner]

The issue bpo-26567 enhanced the warnings module to pass the destroyed object on ResourceWarning. The implementation to log the traceback where the object was allocated is non trivial, so I chose to only implement it in Python.

Problem: _warnings.warn_explicit() is lazy, it tries to get the warnings module from sys.modules, but it uses its own logger if the warnings is not imported yet.

As a consequence, the traceback where the object was allocated is only logged if tracemalloc is tracing Python memory allocations *and* if the warnings module is already imported.

I suggest to modify _warnings.warn_explicit() to import the warnings if it's not imported yet.

There are maybe issues to import a module during Python finalization? A comprise is to not try to import when Python finalization has started.

Example:
---

#import warnings
import tracemalloc
tracemalloc.start(10)
f = open("/etc/issue")
f = None

---

Current output:
---
x.py:5: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
f = None
---

Output if you uncomment the "import warnings":
---
x.py:5: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
f = None
Object allocated at (most recent call first):
File "x.py", lineno 4
f = open("/etc/issue")
---

Attached patch tries to import warnings if source object is set but not after the Pythin finalization has started.

[python-dev]

New changeset 53343d095f78 by Victor Stinner in branch 'default':
_warnings.warn_explicit(): try to import warnings
https://hg.python.org/cpython/rev/53343d095f78