Some quick comments on Martin's pull request.

1. The changes are sufficient to let the user make things work the way it is requested that they work and anyone who does not start using the new format_locals parameter will get the old behavior.

2. A side comment: I just noticed that the docstring for FrameSummary.__init__ does not document the filename, lineno, and name parameters.  Perhaps this could be fixed at the same time?

3. The new parameter format_locals is slightly confusingly named, because it does not format a single string from all the locals but instead gives a dictionary of strings with one string for each local.

4. I personally think it would be better to include something like the example value for format_locals as an extra attribute of the traceback module so everybody doesn't have to write the same function.  That said, the documented example is sufficient.

5. It is not clear to me why this stringify-ing of the locals can't be done in StackSummary._extract_from_extended_frame_gen.  It passes the dictionary of locals and also the function to transform it to FrameSummary.  It would make more sense to transform it first.  I suppose there might be some user somewhere who is directly calling FrameSummary so the interface needs to stay.

6. I fantasize that the new format_locals parameter would have access to the frame object.  In order for this to happen, the frame object would have to be passed to FrameSummary instead of the 3 values (locals, name, filename) that are extracted from it.  I think the current interface of FrameSummary was designed to interoperate with very old versions of Python.  Oh well.

7. If anyone wanted to ever refactor FrameSummary (e.g., to enable my fantasy in point #6 just above), it becomes harder after this.  This change has the effect of exposing details of the interface of FrameSummary to users of StackSummary.extract and TracebackException.  The four parameters that the new parameter format_locals takes are most of the parameters of FrameSummary.