The https://docs.python.org/dev/c-api/init.html documentation lists many functions which is the legacy way to configure the Python initialization. These functions are kept for backward compatibility but have flaws and are less reliable than the new PyConfig API (PEP 587) documented at https://docs.python.org/dev/c-api/init_config.html

I propose to deprecate the legacy functions to configure the Python initialization. Examples:

* Py_SetPath()
* Py_SetProgramName()
* Py_SetPythonHome()
* Py_SetStandardStreamEncoding()
* PySys_AddWarnOption()
* PySys_AddWarnOptionUnicode()
* PySys_AddXOption()

I don't propose to schedule the removal of these functions, only mark them as deprecated in the *documentation*.

Related issue: bpo-43956 "C-API: Incorrect default value for Py_SetProgramName" and PR 24876.

PyConfig API was added to Python 3.8, so it's now widely available. Python 3.8 always switched to security fixes only phase. So IMO it's ok to deprecate the old API now.

"""
Check warning on line 82 in Python/frozenmain.c
GitHub Actions / Ubuntu
‘Py_SetProgramName’ is deprecated [-Wdeprecated-declarations]
"""

Well, Py_FrozenMain() should be rewritten with PyConfig, I want to do that for a long time, but I don't know how to test it. It would be nice to have a very basic test for Py_FrozenMain().

See also:
https://mail.python.org/archives/list/capi-sig@python.org/thread/5QLI3NUP3OSGLCCIBAQOTX4GEJQBWJ6F/

commit 6cd0446ef72c6676b292d7f54b1ddb8ae5e1fb8d
Author: Victor Stinner <vstinner@python.org>
Date:   Wed May 12 23:59:25 2021 +0200

    bpo-44113: Deprecate old functions to config Python init (GH-26060)
    
    Deprecate the following functions to configure the Python
    initialization:
    
    * PySys_AddWarnOption()
    * PySys_AddWarnOptionUnicode()
    * PySys_AddXOption()
    * PySys_HasWarnOptions()
    * Py_SetPath()
    * Py_SetProgramName()
    * Py_SetPythonHome()
    * Py_SetStandardStreamEncoding()
    * _Py_SetProgramFullPath()
    
    Use the new PyConfig API of the Python Initialization Configuration
    instead (PEP 587).

commit a0ccc404ca649c2a1635511a09df2454e47b4d66
Author: Dong-hee Na <donghee.na@python.org>
Date:   Thu May 13 08:22:18 2021 +0900

    bpo-44113: Update __xxtestfuzz not to use Py_SetProgramName (GH-26083)

commit ec7c09b9bc9a8f333a0295b41ea88986c320448c
Author: Victor Stinner <vstinner@python.org>
Date:   Thu May 13 02:27:56 2021 +0200

    bpo-44113: Fix compiler warning in PySys_AddWarnOption() (GH-26084)
    
    Ignore Py_DEPRECATED() warning on purpose.

commit 7565586724692e2ad164d770af9675f7a261fe3a
Author: Dong-hee Na <donghee.na@python.org>
Date:   Thu May 13 10:19:46 2021 +0900

    bpo-44113: Update fromzenmain not to use Py_SetProgramName (GH-26085)

> Well, Py_FrozenMain() should be rewritten with PyConfig, I want to do that for a long time, but I don't know how to test it. It would be nice to have a very basic test for Py_FrozenMain().

I created bpo-44131: [C API] Add tests on Py_FrozenMain().

Thanks for the help Dong-hee ;-) I close the issue, functions are now deprecated.

New changeset a7f8dfd25a167ccfde9996c499fa38a2aba60022 by Victor Stinner in branch 'main':
bpo-44113: Move the What's New entry to Deprecate section (GH-28974)
https://github.com/python/cpython/commit/a7f8dfd25a167ccfde9996c499fa38a2aba60022