Difference in _use_fd_functions in shutil.py and test_shutil.py

[frenzymadness]

Bug report

Bug description:

When switching from os.listdir to os.scandir 7 years ago somebody forgot to update the definition of _use_fd_functions in test_shutil together with shutil module.

in shutil:

_use_fd_functions = ({os.open, os.stat, os.unlink, os.rmdir} <=
                     os.supports_dir_fd and
                     os.scandir in os.supports_fd and
                     os.stat in os.supports_follow_symlinks)

in test_shutil:

        _use_fd_functions = ({os.open, os.stat, os.unlink, os.rmdir} <=
                             os.supports_dir_fd and
                             os.listdir in os.supports_fd and
                             os.stat in os.supports_follow_symlinks)

I believe there is no need to calculate this variable twice so I'm gonna propose a PR where I use the value from shutil.py in its tests. Nobody will ever forget to update both places again.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-119048: Use shutil._use_fd_functions in test_rmtree_uses_safe_fd_v… #157912

[frenzymadness]

As I'm thinking about it more there might be a reason why this is calculated again in the tests and then compared with the value from the shutil module.

But given the current status, this check:

        _use_fd_functions = ({os.open, os.stat, os.unlink, os.rmdir} <=
                             os.supports_dir_fd and
                             os.listdir in os.supports_fd and
                             os.stat in os.supports_follow_symlinks)
        if _use_fd_functions:
            self.assertTrue(shutil._use_fd_functions)

in test_rmtree_uses_safe_fd_version_if_available checks whether both os.listdir and os.scandir are in os.supports_fd because that's the only difference between _use_fd_functions and shutil._use_fd_functions in this context.

Does it make sense to test this?

[frenzymadness]

Ccing @encukou since we were discussing it briefly.

[encukou]

If the difference is intentional, it should have a comment. I don't think it is.

IMO, the repeated code in the test is not useful and we can use shutil._use_fd_functions directly.
@serhiy-storchaka do you agree?

[vasiliyk]

I've opened the PR implementing what @encukou suggested: the test now uses shutil._use_fd_functions directly instead of recomputing it with the stale os.listdir check.