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Bugfix: Ensure that we take a copy of the sys.modules before iterating over it#1514

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pelson wants to merge 1 commit intoipython:mainfrom
pelson:feature/mutable-sys-modules
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Bugfix: Ensure that we take a copy of the sys.modules before iterating over it#1514
pelson wants to merge 1 commit intoipython:mainfrom
pelson:feature/mutable-sys-modules

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@pelson pelson commented May 4, 2026

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I encountered an issue in the CERN Python distribution when running IPykernel, caused by the fact that Python invocations in our distribution trigger a Python thread which does some logging. The issue:

[I 2026-05-04 15:47:50.633 ServerApp] AsyncIOLoopKernelRestarter: restarting kernel (3/5), new random ports
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/tmp/venv26/lib/python3.14/site-packages/ipykernel_launcher.py", line 18, in <module>
    app.launch_new_instance()
    ~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/tmp/venv26/lib/python3.14/site-packages/traitlets/config/application.py", line 1074, in launch_instance
    app.initialize(argv)
    ~~~~~~~~~~~~~~^^^^^^
  File "/tmp/venv26/lib/python3.14/site-packages/traitlets/config/application.py", line 118, in inner
    return method(app, *args, **kwargs)
  File "/tmp/venv26/lib/python3.14/site-packages/ipykernel/kernelapp.py", line 726, in initialize
    self.init_kernel()
    ~~~~~~~~~~~~~~~~^^
  File "/tmp/venv26/lib/python3.14/site-packages/ipykernel/kernelapp.py", line 573, in init_kernel
    kernel = kernel_factory(
        parent=self,
    ...<12 lines>...
        user_ns=self.user_ns,
    )
  File "/tmp/venv26/lib/python3.14/site-packages/traitlets/config/configurable.py", line 583, in instance
    inst = cls(*args, **kwargs)
  File "/tmp/venv26/lib/python3.14/site-packages/ipykernel/ipkernel.py", line 123, in __init__
    m.__file__ for m in sys.modules.values() if hasattr(m, "__file__") and m.__file__
                        ~~~~~~~~~~~~~~~~~~^^
RuntimeError: dictionary keys changed during iteration

Boils down to the fact that sys.modules is a shared, mutable dictionary that can be modified by background threads. Any mutation during iteration raises RuntimeError: dictionary keys changed during iteration on Python 3.14, which tightened its detection of this condition.

Using .copy() the recommended solution as per bugs.python.org/issue40327, so I applied it to one existing location too.

@pelson pelson force-pushed the feature/mutable-sys-modules branch from 93c4dd5 to a5cfbf7 Compare May 4, 2026 15:22
@pelson pelson changed the title Ensure that we take a copy of the sys.modules before iterating over it Bugfix: Ensure that we take a copy of the sys.modules before iterating over it May 4, 2026
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