On the first run all the tests from test_warnings suite pass.
On every subsequent run several tests fail because warnings being emitted via module.warn/module.warn_explicit functions are ignored by default and not caught by the catch_warnings class object.

It turns out that failures are caused by test_warnings._WarningsTests.test_default_action that changes default action for warnings to ignore but fails to restore it back to default.
Even though the code of test_warnings._WarningsTests.test_default_action contains the following finally block which sets module's default action member to its original value, this value is never propagated to the interpreter's state structure.

        finally:
            self.module.defaultaction = original

The quirk is in the broken reference chain.
At the very beginning of the test this chain looks perfectly correct: original -> module.defaultaction -> module._defaultaction -> interp.warnings.default_action.
However, test deletes module.defaultaction and assigns a different string literal (i.e. ignore) to it thus taking module.defaultaction away from the aforementioned chain:

                # Test removal.
                del self.module.defaultaction
                ...
                # Test setting.
                self.module.defaultaction = "ignore"

Furthermore, as soon as a warning is fired after module.defaultaction value has been updated, interp.warnings.default_action is set to reference new module.defaultaction.
The bottom line result is two separate reference chains: original -> module._defaultaction and module.defaultaction -> interp.warnings.default_action.
The assignment self.module.defaultaction = original in the test's finally block (see above) can by no means change interp.warnings.default_action value despite partially restoring the original reference chain: original -> module.defaultaction -> module._defaultaction.

The warnings C module interface is pretty terse containing only two functions: warn_explicit and _filters_mutated.
The latter one has nothing to do with interp.warnings.default_action value whereas the former does exactly what we need: sets interp.warnings.default_action to reference module.defaultaction.

• Issue: test_default_action from test_warnings suite doesn't restore default action in the interpreter's state structure #96661