Support `post_mortem` debugging

[TauPan]

Since pytest supports the --pdb and --pdbcls switches to launch a debugger if a test assertion fails:

  --pdb                 start the interactive Python debugger on errors or
                        KeyboardInterrupt.
  --pdbcls=modulename:classname
                        start a custom interactive Python debugger on errors.
                        For example:
                        --pdbcls=IPython.terminal.debugger:TerminalPdb

it could be useful (at least to me) if debugpy could support this. See emacs-lsp/dap-mode#506

I may be wrong but it looks like debugpy (and vscode) do not support this yet.

Adding this directly in debugpy could be easier than grafting it onto a pytest plugin, as I proposed in emacs-lsp/dap-mode#506

[fabioz]

This is already available (in debugpy it's the Uncaught exceptions breakpoint).

[fabioz]

As a note, for pytest it's not the Uncaught exceptions (because the exceptions are actually caught by pytest), instead, you want the User Uncaught Exceptions -- in this case it's not available by default, but it should appear to you if you set an environment variable such as DEBUGPY_EXCEPTION_FILTER_USER_UNHANDLED=1 and make a launch in VSCode -- when the client will update the exceptionBreakpointFilters.

@karthiknadig @int19h maybe it's time to remove the need for the DEBUGPY_EXCEPTION_FILTER_USER_UNHANDLED environment variable to enable this handling? -- reopening to check whether this should be done...

[int19h]

Yep, we decided to enable the option with the current wording.

[fabioz]

It seems like #724 is tracking that (so, I'm closing this issue).

[wookayin]

I think the User Uncaught Exceptions breakpoint mode is not enough for implementing post-mortem mode, because it will pause too aggressively at exceptions that regularly crosses the boundaries between user and library code (see #1102 for more details). I wrote a feature request in #1103.

[wookayin]

See #723 as well to make debugpy handle an thrown exception