Re: [Pyobjc-dev] sys.excepthook and AppHelper.runEventLoop

[Ronald O. <ron...@ma...>]

On 20 Nov, 2012, at 3:11, Ben Smith <bcs...@gm...> wrote:

> Benjamin Smith <bcsmith05 <at> gmail.com> writes:
> 
>> 
>> Hi all,
>> 
>> I have a python script that I'm bundling with py2app and run using
> AppHelper.runEventLoop.  In my script, I
>> set sys.excepthook to a custom function in order to do some cleanup when there
> is an unhandled exception. 
>> However, because runEventLoop runs the entire runloop inside a try, except
> block, sys.excepthook is
>> never called and the application simply terminates.  It looks like the only
> way to mimic this behavior is to
>> pass in my cleanup code within the 'unexpectedErrorAlert' parameter to
> runEventLoop, but that seemed to
>> not be the best approach.  Is there another way to mimic sys.excepthook that
> I'm missing?
>> 
>> Thanks!
>> Ben
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> 
> Hi again,
> 
> It looks like my understanding earlier was not correct.  The problem was
> unrelated to runEventLoop's try/catch blocks, but rather seemed to be related to
> the fact that my exception was being raised in code that was fired off from
> applicationDidFinishLaunching.  It seems that when python code raises an
> exception within a method called from the run loop, the exception is handled by
> the run loop (generating an objective C exception) and so never reaches the
> python unhandled exception handler.

The Objective-C exception should be converted to a Python one by the bridge,
unless the runloop swallows the exception (and it does that at least for some
exceptions).

> 
> The first thing we tried was to use NSSetUncaughtExceptionHandler to catch the
> objective-C exceptions, but it seems like that function is not implemented.  So
> now what we're going with is to write a decorator to wrap methods that could be
> called from the run loop in try, except, and then on the except, route the
> exception to our sys.excepthook function.  Does that sound reasonable?

That's the best solution. In general Apple's code doesn't like exceptions at all,
throwing exceptions through an Objective-C callstack could result in resource
leaks and other misbehavior. That's because Apple tends to use exceptions only
to signal programmer errors, "normal" errors are usually signalled using return
values and/or NSError values.

You could also call "objc.setVerbose(1)". After that call PyObjC will print exceptions
when converting them to/from Objective-C. 

Ronald
> 
> Thanks a bunch!
> Ben
> 
> 
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