NamedTemporaryFile doesn't issue a ResourceWarning when left unclosed on POSIX

[graingert]

Bug report

Bug description:

import sys
import tempfile


def main():
    tempfile.NamedTemporaryFile()

if __name__ == "__main__":
    sys.exit(main())

when run with python -Werror demo.py nothing happens

if you open a file normally you get a ResourceWarning:

import sys
import tempfile


def main():
    open("example", "w")

if __name__ == "__main__":
    sys.exit(main())

 graingert@conscientious  ~/projects/temp_file_resource_warnings  python -Werror demo2.py
Exception ignored in: <_io.FileIO name='example' mode='wb' closefd=True>
Traceback (most recent call last):
  File "/home/graingert/projects/temp_file_resource_warnings/demo2.py", line 6, in main
    open("example", "w")
ResourceWarning: unclosed file <_io.TextIOWrapper name='example' mode='w' encoding='UTF-8'>

it appears that you do get a ResourceWarning on windows, but I've only seen it in CI I havn't reproduced locally yet

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-126639: Add ResourceWarning to NamedTemporaryFile #126677

[graingert]

I think this __del__ needs to issue the resource warning:

cpython/Lib/tempfile.py

Lines 471 to 472 in 450db61

	def __del__(self):
	self.cleanup()

[graingert]

I don't see how that's related? The resource delete on __del__ is a last resort, and it needs to issue a ResourceWarning so that code that fails to finalise explicitly can be detected

[picnixz]

Ah sorry, I misunderstood the "open again" in the sense that you can leave the descriptor hanging. My bad. However, non-named temporary files say:

It will be destroyed as soon as it is closed (including an implicit close when the object is garbage collected).

Should implicit close emit a warning in this case? Actually, why is there a warning on Windows?

[graingert]

Should implicit close emit a warning in this case?

Yes, all implicit closing of resources on GC should issue a ResourceWarning

[graingert]

I suspect the way to go here is to replace _TemporaryFileCloser with a weakref.finalize

[tomasr8]

I suspect the way to go here is to replace _TemporaryFileCloser with a weakref.finalize

Looks like it, TemporaryDirectory is doing something similar:

cpython/Lib/tempfile.py

Lines 885 to 888 in 450db61

	self._finalizer = _weakref.finalize(
	self, self._cleanup, self.name,
	warn_message="Implicitly cleaning up {!r}".format(self),
	ignore_errors=self._ignore_cleanup_errors, delete=self._delete)

[picnixz]

Categorizing it as a feature as per #126677 (comment).