bpo-36829: Add test.support.catch_unraisable_exception()

[vstinner]

• Copy test_exceptions.test_unraisable() to
  test_sys.UnraisableHookTest().
• test_exceptions.test_unraisable() uses catch_unraisable_exception();
  simplify the test. test_sys now checks the exact output.
• Use catch_unraisable_exception() in test_coroutines,
  test_exceptions, test_generators.

https://bugs.python.org/issue36829

[vstinner]

I chose to leave test_io unchanged on purpose: I have a local branch which fix also https://bugs.python.org/issue36918 in Lib/_pyio.py and use this new catch_unraisable_exception() function. Once this PR will be merged, I will write a second PR based on it.

[vstinner]

It's a little bit strange to have to catch half of the error message from stderr, and the other half from sys.unraisablehook. My PR #13488 will allow to inject the error message in the unraisable exception to catch both at the same time.

[graingert]

this try/with/finally is somewhat tricky, how about:

try:
withsupport.throw_unraisable_exceptions():
...
exceptExceptionase:
... # the exception is now here

eg:

@contextlib.contextmanagerdefthrow_unraisable_exceptions():
unraisable=Noneold_hook=sys.unraisablehookdefhook(exc):
nonlocalunraisableunraisable=excsys.unraisablehook=hooktry:
yieldifunraisableisnotNone:
raiseunraisablefinally:
unraisable=Nonesys.unraisablehook=old_hook

[graingert]

or recommend usage like this:

withsupport.catch_unraisable_exceptions() ascm:
...
cm.unraisable# only available hereassertcm.unraisableisNone# now it's gone

by updating __exit__:

def__exit__(self, *exc_info):
self.unraisable=None# clear unraisable heresys.unraisablehook=self._old_hook

[vstinner]

Good idea :-) I implemented your __exit__ idea to avoid the need for try/finally.

throw_unraisable_exceptions() might be useful, but modified tests needs to access the 'obj' attribute of the unraisable hook. Later, they might also want to get access to the 'err_msg' attribute: #13488

[graingert]

There's still a sharp edge here:

how about making cm.unraisable throw if accessed outside the context instead of being None, eg:

def __exit__(self, *exc_info):
del self.unraisable
sys.unraisablehook = self._old_hook

[graingert]

and if you want to access additional unraisablehook info using throw_unraisable_exceptions:

classUnraisableException(Exception):
def__init__(self, unraisable):
self.unraisable=unraisablesuper().__init__(self, unraisable)
@contextlib.contextmanagerdefthrow_unraisable_exceptions():
unraisable=Noneold_hook=sys.unraisablehookdefhook(unraisable_):
nonlocalunraisableunraisable=unraisable_sys.unraisablehook=hooktry:
yieldifunraisableisnotNone:
raiseUnraisableException(unraisable) fromunraisable.exc_valuefinally:
sys.unraisablehook=old_hook

then you can use it with:

try:
withthrow_unraisable_exceptions():
...
exceptUnraisableExceptionase:
print(repr(e.__cause__))
err_msg=e.unraisable.err_msg

[vstinner]

I wrote PR #13554 to implement your "del self.unraisable" idea.