The default unraisablehook implementation does not print chained exceptions or recurse into exception groups

[graingert]

here's a demo in the repl

>>> class Foo:
...     def __del__(self):
...         raise BaseExceptionGroup("the bad", [Exception("critical debug advice here")])
... 
>>> f = Foo()
>>> del f
Exception ignored in: <function Foo.__del__ at 0x7f2f59df2200>
Traceback (most recent call last):
  File "<stdin>", line 3, in __del__
ExceptionGroup: the bad (1 sub-exception)
>>> 

@vstinner @iritkatriel I'm not sure if this is an omission or by design, but if an exception with critical debug advice is raised in an exceptiongroup during __del__ I can't see the type or error message

I expected the output to include:

  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 1, in <module>
  | ExceptionGroup: the bad (1 sub-exception)
  +-+---------------- 1 ----------------
    | Exception: critical debug advice here
    +------------------------------------

[iritkatriel]

Looks like the default unraisable hook has its own implementation of exception printing. Why isn't it just calling _PyErr_Display?

[graingert]

looks like the threading.excepthook does the right thing:

>>> t = threading.Thread(target=raise_, args=(BaseExceptionGroup("the bad", [Exception("critical debug advice here")]),))
>>> t.start()
Exception in thread Thread-4 (raise_):
  + Exception Group Traceback (most recent call last):
  |   File "/usr/lib/python3.11/threading.py", line 1038, in _bootstrap_inner
  |     self.run()
  |   File "/usr/lib/python3.11/threading.py", line 975, in run
  |     self._target(*self._args, **self._kwargs)
  |   File "<stdin>", line 1, in raise_
  | ExceptionGroup: the bad (1 sub-exception)
  +-+---------------- 1 ----------------
    | Exception: critical debug advice here
    +------------------------------------
>>> t.join()
>>> 

[graingert]

Looks like the default unraisable hook has its own implementation of exception printing. Why isn't it just calling _PyErr_Display?

if this is a bug in unraisable hook can we get the fix backported to 3.10 so the exceptiongroup backport can use it?

[iritkatriel]

I don't know if this is a bug or an enhancement. In any case it's hookable, right?

[iritkatriel]

Looks like the default unraisable hook has its own implementation of exception printing. Why isn't it just calling _PyErr_Display?

if this is a bug in unraisable hook can we get the fix backported to 3.10 so the exceptiongroup backport can use it?

I don't think back porting this even could affect the exception group backport - in 3.10 _PyErr_Display() doesn't know anything about exception groups.

[iritkatriel]

The default unraisablehook doesn't print chained exceptions either.

I think this may be deliberate to avoid recursion when the interpreter could be in a delicate state (e.g. under memory pressure). This is definitely not a bugfix. I'm removing 3.11.