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gh-125893: Add type check for category argument in warnings.simplefilter and warnings.filterwarning #136305

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gh-125893: Add type check for category argument in warnings.simplefilter and warnings.filterwarning #136305

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@hasrat17 hasrat17 commented Jul 4, 2025

This PR addresses gh-125893 by adding a type check for the category argument in warnings.simplefilter and warnings.filterwarning,

Previously, warnings.filterwarnings correctly raised an error when the category argument was not a class, but warnings.simplefilter accepted invalid types without raising any error. This inconsistency could lead to confusion and improper warning filtering.

Both warnings.simplefilter and warnings.filterwarning were not raising any warnings while ran with python -O but now this is also handled.

$ ./amd64/python.exe -O -c "import warnings; warnings.resetwarnings(); warnings.simplefilter('default', category='Hello');  print(warnings.filters)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\hasra\OneDrive\Documents\opensource\cpython\Lib\warnings.py", line 182, in simplefilter
    raise TypeError("category must be a Warning subclass, "
TypeError: category must be a Warning subclass, not 'str'

./amd64/python.exe -O -c "import warnings; warnings.resetwarnings(); warnings.filterwarnings('ignore', category=Warning); warnings.filterwarnings('ignore', category='Hello'); print(warnings.filters)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\hasra\OneDrive\Documents\opensource\cpython\Lib\warnings.py", line 149, in filterwarnings
    raise TypeError("category must be a Warning subclass, "
TypeError: category must be a Warning subclass, not 'str'

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bedevere-app bot commented Jul 4, 2025

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

@bedevere-app bedevere-app bot mentioned this pull request Jul 4, 2025
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@brianschubert brianschubert mentioned this pull request Jul 4, 2025
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@ZeroIntensity ZeroIntensity self-requested a review July 9, 2025 00:44
ZeroIntensity
ZeroIntensity reviewed Jul 9, 2025
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Sorry for the delay on the review. Please add a test case to test_warnings. You can refer to the devguide if you're unsure how to do that.

Misc/ACKS Outdated
@@ -2149,5 +2149,6 @@ Jelle Zijlstra
Gennadiy Zlobin
Doug Zongker
Peter Åstrand
Hasrat Ali Arzoo
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We try to keep this sorted by last name. You should be closer to the top.

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Oh, also, please add a news entry.

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python-cla-bot bot commented Jul 10, 2025 •
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@hasrat17
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Thanks for your review! @ZeroIntensity I've addressed all the feedback and updated accordingly.

@hasrat17 hasrat17 requested a review from ZeroIntensity July 10, 2025 15:32
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Hi @ZeroIntensity
I’ve addressed all the review feedback and updated the branch. Could you please take a look at this PR or assign it to the appropriate reviewer? Thank you!

ZeroIntensity
ZeroIntensity reviewed Jul 24, 2025
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Misc/NEWS.d/next/Windows/2025-07-10-19-28-11.gh-issue-125893.lD8Nyd.rst
@@ -0,0 +1,2 @@
Added validation for the ``category`` argument in
``warnings.filterwarnings()`` and ``warnings.simplefilter()``
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Sphinx markup:

Suggested change
``warnings.filterwarnings()`` and ``warnings.simplefilter()``
:func:`warnings.filterwarnings()` and :func:`warnings.simplefilter()`.

Lib/_py_warnings.py
if isinstance(category, type) and issubclass(category, Warning):
return True
if isinstance(category, tuple):
if __debug__:
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I'd still consider the isinstance checks above to be expensive. Could you move this __debug__ check to before any calls to isinstance?

Lib/test/test_warnings/__init__.py
@@ -398,6 +398,33 @@ def test_argument_validation(self):
with self.assertRaises(ValueError):
self.module.simplefilter('ignore', lineno=-1)

def test_invalid_category_types(self):
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We probably need to skip this if we're not in debug mode, so add something like @unittest.skipUnless(__debug__, ...).

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