Hi,

When iterating over sys.modules it might be that because of lazy loading or that other module reload is being reloaded (even if sys.modules.copy() is being used) you will get:

"RuntimeError: dictionary changed size during iteration"

The usage of sys.modules and iteration over it is used in many places

I know that this is a known issue but didn't find any solution or fix for that. Also, if there is some work around that might solve this, I would appreciate it.

Thanks,
Idan

One standard way of preventing this is copying the dictionary whenever there's risk of it changing out from under you, as in:

modules = sys.modules.copy()
for key, value in modules.items():
    ...

Do you have an example for which "for m in sys.modules: print(m, end=' ')" fails?  In particular, is there code in the stdlib that fails iterating over sys.modules?  If not the latter, this should be closed as 'Not a bug'.

Note that questions about using Python should go elsewhere, such as python-list or stackoverflow.

An example can be found here: https://github.com/python/cpython/commit/7058d2d96c5ca4dfc6c754c5cd737c6eb2a8fd67
https://bugs.python.org/issue13487

Those links are about an issue that was until March 2020 with how sys.modules is iterated. It is not safe to iterate over since it can get updated during the iteration because of lazy importing for example.

If you look in older reports you can find that there us an issue with iterating over it and it was not addressed yet at all.

In #13487, Gregory fixed the problem by using .copy().  That seems to have worked for 1 1/2 years.  You still have not reported an actual bug in the current CPython stdlib.

Perhaps we should mention in https://docs.python.org/3/library/sys.html#sys.modules that sys.modules can be unexpectedly changed during iteration by lazy imports or other threads, so that a copy might be needed.

Gregory, what do you think?

New changeset 3d1ca867ed0e3ae343166806f8ddd9739e568ab4 by Gregory P. Smith in branch 'main':
bpo-45353: Remind sys.modules users to copy when iterating. (GH-28842)
https://github.com/python/cpython/commit/3d1ca867ed0e3ae343166806f8ddd9739e568ab4

While arguably unnecessary as it is documented as a dictionary and this is a normal Python dict behavior, it is a global dict and it can be modified at times that are unintuitive to users of all experience levels.  A note in the documentation makes sense.

New changeset 459a4db5eae1f5ef063b34c61cc099820aa9ed0a by Miss Islington (bot) in branch '3.10':
bpo-45353: Remind sys.modules users to copy when iterating. (GH-28842)
https://github.com/python/cpython/commit/459a4db5eae1f5ef063b34c61cc099820aa9ed0a