bpo-40275: Add os_helper submodule in test.support

[shihai1991]

Import the following vars, functions, classes in os_helper:

• rmtree()
• unlink()
• rmdir()
• FS_NONASCII
• TESTFN_UNICODE, TESTFN_NONASCII, TESTFN_UNENCODABLE, TESTFN_UNDECODABLE
• SAVEDCWD
• temp_dir()
• change_cwd()
• temp_cwd()
• temp_umask()
• create_empty_file()
• make_bad_fd()
• EnvironmentVarGuard
• can_symlink(), skip_unless_symlink()
• can_xattr(), skip_unless_xattr()
• fs_is_case_insensitive()
• findfile
• fd_count()
• FakePath

https://bugs.python.org/issue40275

[shihai1991]

@vstinner Hi, victor. Pls have a look again when you have free time.

[vstinner]

For the first PR (this one), please leave support/__init__.py unchanged: only import names in os_helper. Also, please leave test_xxx.py unchanged.

[vstinner]

For example, os_helper.py would contain something like:

from test.support import TESTFN
...

[shihai1991]

For example, os_helper.py would contain something like:

from test.support import TESTFN
...

Oh, My logic is the opposite. IMHO, it is same behavior, no?

[vstinner]

Brett and me explained to you that a PR modifying 100+ files is a bad idea.

Adding a new submodule which only imports existing functions avoids modifying any existing test and so it becomes possible to convert tests slowly, one by one, or 20 by 20.

[shihai1991]

Brett and me explained to you that a PR modifying 100+ files is a bad idea.

Pls forgive me, I have relealized that it's not easy to review and noisy too many developers.

Adding a new submodule which only imports existing functions avoids modifying any existing test and so it becomes possible to convert tests slowly, one by one, or 20 by 20.

IMO: a new submodule which only imports existing functions same as move existing functions in new submodule and reimport those existing functions in test.support

Oh, wait. You are right. Some testcase use inner functions of test.support will be affected.

[vstinner]

move existing functions in new submodule and reimport those existing functions in test.support

Right. I tried to check if there are remaining functions in support/init.py which should be moved to os_helper. But since functions are not moved, it's not easy.

Would you mind to create a second PR which moves functions but adds aliases in support/init.py? It sounds like an even better approach.

[vstinner]

Side note: I'm not sure why it exists in test.support. It seems like only test.libregrtest uses it. Maybe it should be moved (later) to test.libregrtest.

[shihai1991]

Hm, I checked this detail. Looks like the describe info is right. It points that SAVEDCWD=os.getcwd()

[vstinner]

IMO PR #20765 is the right way to fix this issue.