Allow relative imports in the `__main__` module

Feature or enhancement

Proposal:

Expanding on https://github.com/python/cpython/issues/109853#issuecomment-2076845977

@zooba, were you envisioning to do this for all scenarios where a __main__ module is exists? I think that makes sense for consistency, but we may want to consider all scenarios to make sure the UX makes sense.

AFAICT, these should be all of them:

(this refers to relative imports in the same directory, it does not cover relative imports to the parent directory)

1. Running a package module via python -m <module>
   • Relative imports already work here, no changes needed.
2. Running a source or extension module via python -m <module>
   • Relative imports don't work here.
   • Allowing relative imports here feels a bit weird, especially if we were to enable -P by default. For it to run, the module would have to be in sys.path, meaning it probably installed rather than being provided by the user, so there isn't much necessity for relative imports.
3. Running a package module directly (python <module.zip>)
   • Relative imports already work here, no changes needed.
4. Running a file directly (python <script.py>)
   • Relative imports don't work here.
5. Running a command string via python -c <command>
   • Relative imports don't work here.
   • There's no canonical base directory to use as a reference, should we use the current directory?
6. Running a command string via the stdin
   • Relative imports don't work here.
   • There's no canonical base directory to use as a reference, should we use the current directory?
7. Running the repl
   • Relative imports don't work here.
   • There's no canonical base directory to use as a reference, should we use the current directory?

I think 4), 5), 6), and 7) are the only use-cases that call for relative imports to work. 2) feels a bit weird, but it may be worth supporting for consistency.

I played a bit with the code, and the implementing this doesn't seem much difficult. I was able to write a working PoC targeting only 4) with much difficulty, so I don't think that's a worry.

Considering this, do we think it makes sense to make this change?
If so, should it cover all scenarios, or do we want to keep some of them as-is?

For further reflection, how much of an actual improvement would this actually be over the current behavior of adding the current directory to sys.path?
I can't help but feel a bit like we are trading one weird behavior for another.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

まず issue 109853 にリンクされた議論を読み、ここに列挙されている main の 7 つの実行シナリオを比較します。特に、直接ファイル、-c、stdin、REPL を比較してください。どのケースで相対インポートをサポートすべきか、正規の基準が存在しない場合にどのディレクトリを基準とすべきか、また現在のディレクトリを sys.path に追加することで動作が改善するかを判断します。実装前に、サポート対象の範囲と、それによって得られるユーザー体験について合意できていれば完了です。