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Recently, the Python initialization was reworked to start to implement the PEP 432:
- bpo-22257: commits 1abcf6700b4da6207fe859de40c6c1bada6b4fec and 6b4be195cd8868b76eb6fbe166acc39beee8ce36
Now, Py_NewInterpreter() leaks a reference on warnoptions in _PySys_EndInit(). We tried with Stéphane Wirtel and Louie Lu to add Py_DECREF(warnoptions), but test_capi does crash with this change.
The problem is that warnoptions is stored in a C global variable *and* in sys.warnoptions of each interpreter. The ownership of this variable is unclear.
I don't think that it's a good idea to share a list between two interpreters and so I created this issue to propose to redesign this variable.
The tricky part is that the C global variable is also accessed by 2 public C functions: PySys_ResetWarnOptions() and PySys_AddWarnOptionUnicode().

I'd suggest that the right fix here would be to move warnoptions into the config struct as proposed in the PEP: https://www.python.org/dev/peps/pep-0432/#supported-configuration-settings
The main reason we merged this as a private API was so we could do that setting-by-setting, with the test suite ensuring we weren't breaking anything.
It looks like in this case, it's the status quo that's broken, and the change makes it possible to fix it :)

See also bpo-30547 for other refleaks somehow related to that one.

New changeset 865de27dd79571a4a5c7a7d22a07fb909c4a9f8e by Victor Stinner in branch 'master':
bpo-30598: _PySys_EndInit() now duplicates warnoptions (#1998)
https://github.com/python/cpython/commit/865de27dd79571a4a5c7a7d22a07fb909c4a9f8e

Nick: "I'd suggest that the right fix here would be to move warnoptions into the config struct as proposed in the PEP: https://www.python.org/dev/peps/pep-0432/#supported-configuration-settings"
Nick, Eric: do you want to do that it? If you are busy, that's fine, I will just close the issue since my concern (the ref leak) is now fixed.

We'll likely still move it eventually, but I don't think that's a good reason to keep this issue open - it's more a part of making incremental progress towards being able to make PEP 432 a public API.

This bug came back in master: see bpo-31420.