Currently, traceback objects don't expose any public constructor, are immutable, and don't have a __dict__ or allow subclassing, which makes it impossible to add extra annotations them.

It would be nice if these limitations were lifted, because there are rare but important cases where code needs to manipulate tracebacks directly, and currently these have to use awful stuff like ctypes. For example:

Jinja2:
https://github.com/pallets/jinja/blob/bec0065c4e7e89e7d893eda7840ba0219824b23c/jinja2/debug.py#L298-L372

Trio:
https://github.com/python-trio/trio/blob/496493afecc22d7d1a17175b6a2748a9c3510066/trio/_core/_multierror.py#L233-L318

(Notice that on PyPy there are no ctypes, but instead they have special extension that's supported for this case only to keep jinja2 working.)

For the above cases, what's needed is the ability to instantiate and assign to the fields of traceback objects.

In addition, in trio I'd like to be able to annotate traceback objects so that the traceback printing machinery can do things like hide "internal" tracebacks, or highlight places where exceptions jumped between tasks. This would be much easier if there were some way to attach data to tracebacks. Probably it doesn't make sense for traceback objects to have a __dict__ by default for speed/memory reasons, but we could add a dedicated metadata slot that is normally empty but can have arbitrary data assigned, or if they allowed subclassing then I could add a __dict__ in a subclass

I'm CC'ing the "import machinery" interest list, because my understanding is that with the new import system there's a similar desire to hide "internal" traceback frames, and while the features requested in this bug report won't solve that problem directly they might (are intended to) provide some useful machinery for it. Feel free to un-CC if I'm wrong :Jinja2:
https://github.com/pallets/jinja/blob/bec0065c4e7e89e7d893eda7840ba0219824b23c/jinja2/debug.py#L298-L372
-).

I think that this should be very straightforward to implement: it's just that no-one ever implemented setters etc. for tracebacks.