Bug: QApplication() raise nothing on tty

[Florian Bruhin]

Hey,

> I presume this problem is the same as the one that caused NeXpy to
> start crashing starting with PyQt 5.5 (see
> https://github.com/nexpy/nexpy/commit/b19480a37e1d14278945672c08cab614a87b15eb).
> This seemed to be, in my opinion, an unfortunate design decision by Qt
> to abort a process whenever an error was encountered, in a way that
> was inaccessible to Python exception handling.

That's about what happens when there's an unhandled Python exception
from Python called from C++ code.

It won't change anything about what happens when Qt calls abort()
(via qFatal()).

> I think there could be an argument for PyQt providing a default way to
> catch these exceptions by defining its own sys.excepthook function,
> but it might be difficult to guarantee that such a function doesn’t
> interfere with other applications that define their own. Perhaps it
> could be an argument when generating the QApplication instance.

This behavior is coming from PyQt in the first place - it checks if a
sys.excepthook is defined, and if not, it aborts.

This is needed because, if Python is called from C++, and an exception
happens, there are only two ways for PyQt to get out of that situation:

1) Print the exception to stdout, and return some kind of default value
to C++ (e.g. an empty string, or 0, or ...). This is what PyQt did
before 5.5, but it lead to people missing exceptions and all kinds of
funny undefined behavior (since PyQt basically just made up a value to
return).

2) abort the process, which is what it does since 5.5.

Yes, 2) is somewhat annoying. But it's the safer choice as 1), which
basically amounts to "silently ignore all exceptions and do something
random instead". Thus you basically need to opt into it by setting a
sys.excepthook.

Florian
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