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().

I am also running into this problem. I'm not 100%, but I'm pretty sure that looping over sys.modules and accessing __warningregistry__ on each module triggers one of my module's __getattr__ functions (PEP 562), which in turn uses setuptools entry points to import an arbitrary set of other modules.

Bizarrely, however, I cannot reproduce on macOS, only Linux. Presumably there's something platform dependent about how the default unittest test runner orders my tests, and hence which modules have already been loaded by the time I call assertWarnsRegex.