PYTHONBREAKPOINT should ignore non-importable breakpoints

Feature or enhancement

Proposal:

I'd like to use ipdb rather than the built-in pdb by default for my breakpoints. However, if I set PYTHONBREAKPOINT="ipdb.set_trace" as a global shell environment variable, and the project does not have ipdb available, then all breakpoints are disabled:

/Users/tomforbes/.../cli.py:144: RuntimeWarning: Ignoring unimportable $PYTHONBREAKPOINT: "ipdb.set_trace"
  breakpoint()

IMO this isn't very user-friendly and can be quite annoying: I need to remember to export it on a per-project or per-shell basis. It would be great to fall back to the standard pdb breakpoint handler if the import fails.

If this is a compatability issue, perhaps a new PYTHONDEFAULTBREAKPOINT env variable could be added (and that can be set globally), which is used if available and falls back to pdb if not?

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

まず、breakpoint() が呼び出されたときに PYTHONBREAKPOINT がどのように処理されるかを追跡します。インポートできないハンドラーに対する既存の RuntimeWarning も含めます。利用できない ipdb.set_trace のフォールバック動作を、標準の pdb ハンドラーを維持したまま定義し、インポート可能なブレークポイントハンドラーと利用できないブレークポイントハンドラーの両方をカバーするテストを追加します。