[issue45836] unittest.TestCase.assertWarns raises RuntimeEror if sys.modules changes size (Python 3.9)

[Łukasz Langa]

Łukasz Langa  added the comment:

Thanks, Dennis! ✨  ✨

--
nosy: +lukasz.langa
resolution:  -> fixed
stage:  -> resolved
status: open -> closed

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[issue45836] unittest.TestCase.assertWarns raises RuntimeEror if sys.modules changes size (Python 3.9)

[Dennis Sweeney]

Dennis Sweeney  added the comment:

https://github.com/python/cpython/pull/29605 was just opened as a backport

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nosy: +Dennis Sweeney

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[issue45836] unittest.TestCase.assertWarns raises RuntimeEror if sys.modules changes size (Python 3.9)

[Patrick Bourke]

New submission from Patrick Bourke :

Hi all,

Forgive me if this is not the correct way to report this, but we have run into 
the issue from #29620 ( https://bugs.python.org/issue29620 ) on Python 3.9.

The fix appears to be present in the tip of 3.8:

https://github.com/python/cpython/blob/c37a0d9c0ae4aa0d9135fac9a58afc7b34ff71d6/Lib/unittest/case.py#L254

and 3.10:

https://github.com/python/cpython/blob/0ef308a2890571c850c624fb99ac00f8951363c6/Lib/unittest/case.py#L255

but missing from 3.9:

https://github.com/python/cpython/blob/0ef308a2890571c850c624fb99ac00f8951363c6/Lib/unittest/case.py#L255

Here is our test case:

import unittest
import graspologic


class MyTestCase(unittest.TestCase):
def test_something(self):
with self.assertWarns(UserWarning):
pass


if __name__ == '__main__':
unittest.main()

fails with:

==
ERROR: test_something (__main__.MyTestCase)
--
Traceback (most recent call last):
  File 
"C:\Users\pbourke\AppData\Roaming\JetBrains\PyCharm2021.2\scratches\test.py", 
line 7, in test_something
with self.assertWarns(UserWarning):
  File 
"C:\Users\pbourke\.pyenv\pyenv-win\versions\3.9.7\lib\unittest\case.py", line 
255, in __enter__
for v in sys.modules.values():
RuntimeError: dictionary changed size during iteration


The module triggering the error appears to be IPython.utils.io, which is 
included somewhere in our dependencies.

My CPython is:
Python 3.9.7 (tags/v3.9.7:1016ef3, Aug 30 2021, 20:19:38) [MSC v.1929 64 bit 
(AMD64)]

--
components: Library (Lib)
messages: 406516
nosy: pbourke
priority: normal
severity: normal
status: open
title: unittest.TestCase.assertWarns raises RuntimeEror if sys.modules changes 
size (Python 3.9)
type: behavior
versions: Python 3.9

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