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

首先跟踪调用 breakpoint() 时 PYTHONBREAKPOINT 的处理方式，包括针对无法导入的 handler 现有的 RuntimeWarning。定义 ipdb.set_trace 不可用时的回退行为，同时保留标准的 pdb handler，并为可导入和不可用的 breakpoint handler 都添加覆盖测试。