New submission from kernc:
If any of the objects in sys.modules is a module-like object that performs some
additional imports in its __getattribute__ (as appropriate) handler, the
following simple unit test test case:
import unittest
import warnings
... # Ensure one of the imported modules is a module-like object as above
class Case(unittest.TestCase):
def test_assertWarns(self):
with self.assertWarns(UserWarning):
warnings.warn('Some warning')
fails with:
======================================================================
ERROR: test_assertWarns (example.Case)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/tmp/example.py", line 9, in test_assertWarns
with self.assertWarns(UserWarning):
File "/usr/lib/python3.4/unittest/case.py", line 205, in __enter__
for v in sys.modules.values():
RuntimeError: dictionary changed size during iteration
----------------------------------------------------------------------
The problem is in the iteration over sys.modules in
unittest.case._AssertWarnsContext.__enter__() and accessing every module's
__warningregistry__ attribute. On this access, the module-like objects may
perform arbitrary actions including importing of further modules which extends
sys.modules.
https://github.com/python/cpython/blob/16ea19fc6653ee4ec1be7cd0206073962119ac08/Lib/unittest/case.py#L226-L227
The simple proposed fix with no foreseen side-effects is to wrap
sys.modules.values() call as a tuple().
----------
components: Library (Lib)
messages: 288370
nosy: kernc
priority: normal
severity: normal
status: open
title: unittest.TestCase.assertWarns raises RuntimeEror if sys.modules changes
size
type: behavior
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue29620>
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