gh-153967: handle invalid file object in argparse._print_message

[ptim0626]

The fix for the above issue, which raises a ValueError if an invalid file is explicitly passed to argparse.print_usage and argparse.print_help. Tests added.

• Issue: argparse _print_message silently fails with invalid file object #153967

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[python-cla-bot]

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[ZeroIntensity]

This is also a user-facing change; please add a news entry.

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[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

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[ptim0626]

This is also a user-facing change; please add a news entry.

Thanks, and this was added.

[ptim0626]

Now I have no idea why test_ctypes failed in the CI...

[ZeroIntensity]

That would be #154106. I just updated the branch, which should fix it.

[ptim0626]

The merge fixed the test_ctypes but the sanitizer build failed with test_external_inspection.

[chris-eibl]

No worries. test_external_inspection is a known flaky test. Also the readthedocs hiccuped - I hit "update branch" to trigger CI again and hope for the best :)

[savannahostrowski]

Could we continue to suppress OSError? If writing to sys.stderr raises OSError during ArgumentParser.exit(), the exception escapes before _sys.exit(status) is reached, changing the exception from SystemExit to OSError. We should also add a test for this, probably.

Suggested change

	file.write(message)
	try:
	file.write(message)
	except OSError:
	pass

[ptim0626]

Thanks for catching this so the existing behaviour is kept. I have put a test and made the change accordingly.

One issue of catching OSError inside _print_message is that if, e.g. a non-writable file object, is passed to print_usage/print_help, it will silently fail. The users may benefit from getting a clearer message about what's gone wrong if an error message io.UnsupportedOperation: not writable is emitted (an example of passing a non-writable file). A lot of file-related exceptions are inherited from OSError such as PermissionError etc. Instead of putting the try ... except inside _print_message, could we do

    def exit(self, status=0, message=None):
        if message:
            try:
                self._print_message(message, _sys.stderr)
            except OSError:
                pass
        _sys.exit(status)

This will:

• ensure argparse.exit still raises SystemExit when sys.stderr raises OSError
• give clearer exception message when invalid file object (e.g. permission issue, non-writable, wrong file path etc) is passed to print_usage/print_help and not silently failed

Happy to keep the current state if it is more appropriate.