Replace MultiError with BaseExceptionGroup

[agronholm]

This PR has two dependencies:

• Drop Python 3.6 support #2210 (required)
• Drop dependency on async_generator #2212 (recommended but not strictly necessary) (skipped)

Closes #2211.
Closes #2427.

[codecov]

Codecov Report

Merging #2213 (80889b2) into master (af66bcf) will increase coverage by 0.01%.
The diff coverage is 99.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2213      +/-   ##
==========================================
+ Coverage   98.98%   99.00%   +0.01%     
==========================================
  Files         119      117       -2     
  Lines       16165    16119      -46     
  Branches     3101     3122      +21     
==========================================
- Hits        16001    15958      -43     
+ Misses        114      113       -1     
+ Partials       50       48       -2     

Impacted Files	Coverage Δ
trio/_core/__init__.py	100.00% <ø> (ø)
trio/tests/test_highlevel_open_tcp_listeners.py	98.25% <75.00%> (-1.75%)	⬇️
trio/__init__.py	100.00% <100.00%> (ø)
trio/_core/_multierror.py	100.00% <100.00%> (ø)
trio/_core/_run.py	100.00% <100.00%> (ø)
trio/_core/tests/test_multierror.py	100.00% <100.00%> (ø)
trio/_core/tests/test_run.py	100.00% <100.00%> (ø)
trio/_highlevel_open_tcp_listeners.py	100.00% <100.00%> (ø)
trio/_highlevel_open_tcp_stream.py	97.53% <100.00%> (+0.19%)	⬆️
trio/tests/test_highlevel_open_tcp_stream.py	100.00% <100.00%> (ø)
... and 3 more

[agronholm]

This needs discussion on whether to drop MultiError cold turkey or to have it inherit from BaseExceptionGroup. It's not yet certain if inheriting it is possible in the first place.

[agronholm]

Here are my thoughts about deprecating MultiError. If we were to deprecate it and have it inherit from BaseExceptionGroup instead, what would be the migration path? There are two cases how it might be used by downstream users:

1. Raising them
2. Catching them (or their sub-exceptions)

To maintain backwards compatibility, both use cases would have to be covered. Trio itself would have to raise MultiError with warning suppression turned on to counter the deprecation warning we would have have its constructor emit. Then we would have to have the extra class methods (catch() and filter()) raise deprecation warnings as well. This only leaves one hole: cases where users catch MultiError directly. We can't emit a deprecation warning from the exceptions property because that would also happen for the users doing the right thing (catching BaseExceptionGroup instead).

Either way, a migration guide would probably be in order.

[smurfix]

Meh. Frankly I don't think we can achieve 100% backwards compatibility, which is especially bad since the affected code is on code paths that are typically not tested very well.

Thus I'd advise to spend the effort on a good migration guide that covers all the bases, preferably from multiple angles. Code that wants to be able to handle both BaseExceptionGroup and MultiError will just have to duplicate the affected error handlers for a year or two.

[agronholm]

Meh. Frankly I don't think we can achieve 100% backwards compatibility, which is especially bad since the affected code is on code paths that are typically not tested very well.

Thus I'd advise to spend the effort on a good migration guide that covers all the bases, preferably from multiple angles. Code that wants to be able to handle both BaseExceptionGroup and MultiError will just have to duplicate the affected error handlers for a year or two.

This seems to be the consensus (on Gitter as well). I will attempt to write a migration guide somewhere in the docs.

[agronholm]

I amended the documentation and the news fragment to point users in the right direction. While this is not strictly a "How to migrate from MultiError to BaseExceptionGroup", trio users tend to be more tech savvy than the average Python user. I hope that is enough.

[agronholm]

In terms of the ideal long-run state, I agree with you. In terms of the actual experience of our users when this is released, dropping the IPython monkeypatch is a major regression: exception group tracebacks will silently show only some of the frames. I think we need to carry that monkeypatch until IPython actually upstreams proper exception group support.

I've restored the IPython custom error handler, even though I still feel this is the wrong place for it. It also now supports (Base)ExceptionGroup.

[oremanj]

I've restored the IPython custom error handler, even though I still feel this is the wrong place for it.

Thanks. Hopefully IPython will implement proper EG support (including the backport) and we can remove Trio's patching of it soon.

Even though the Ubuntu wiki states that Apport is disabled by default? And what warning would they get?

I don't have easy access to an Ubuntu machine right now, but IIRC ubuntu installs a custom sys.excepthook unconditionally, which then calls the default one unless Apport is enabled. But it still looks like a custom excepthook so Trio won't install its own, so you don't get MultiError tracebacks.

I guess the exceptiongroup backport doesn't print a warning like Trio previously would if it fails to patch sys.excepthook. So users won't get a confusing warning... instead, they'll silently fail to see the "multi" parts of the traceback, which is even worse UX. I think that warning is fairly important in helping people understand what's going on.

[agronholm]

I guess the exceptiongroup backport doesn't print a warning like Trio previously would if it fails to patch sys.excepthook. So users won't get a confusing warning... instead, they'll silently fail to see the "multi" parts of the traceback, which is even worse UX. I think that warning is fairly important in helping people understand what's going on.

I'm running Ubuntu on my home server and I can confirm that outside of a virtualenv, the apport hook is indeed present and will prevent exceptiongroup from installing its own hook. So, to not degrade the trio UX, I will fix this by adapting the old trio apport exceptionhook.

[agronholm]

Code-wise this PR should be fine now.

[pquentin]

Thanks for your hard work, @agronholm and @oremanj! We're lucky to have you. I'm really excited!

Will release this tomorrow.