[issue18383] test_warnings modifies warnings.filters when running with "-W default"

[Alex Shkop]

Alex Shkop added the comment:

The issue occurs only if C implementation of _warnings is available. It is caused by re-use of global filters variable from _warnings when warnings is being re-imported.
So warnings modifies _warnings.filters on first import. Then, when you import warnings again it uses already modified version of _warnings.filters and adds same filter for the second time.

I attach a simple patch that prevents duplicates in filters. I'm not sure if this is a best way to fix the problem, but it works. It certainly affects performance of filterwarnings(), but there shouldn't be a lot of filters anyway.

Making filters a set() will solve the problem also, but will require API changes. Although I didn't find anywhere in documentation that warnings.filters should be a list, someone might rely on its .append() method.

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nosy: +ashkop
Added file: http://bugs.python.org/file38301/issue18383_remove_dups.patch

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