Re: [Pyobjc-dev] Return value "handler" / NSEvent.addLocalMonitorForEventsMatchingMask_handler_

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

> On 1 Dec 2017, at 18:30, Just van Rossum <jus...@gm...> wrote:
> 
> From a running PyObjC/Cocoa app, I try to do the following:
> 
> 
> from AppKit import NSKeyDownMask, NSEvent
> 
> def myHandler(event):
>   print(event)
>   return event
> 
> mask = NSKeyDownMask
> monitor = NSEvent.addLocalMonitorForEventsMatchingMask_handler_(mask, myHandler)
> 
> 
> (one could call this in an applicationDidFinishLaunching_() app delegate method)
> 
> 
> The handler indeed gets called when I press a key, but the caller complains it’s returning None:
> 
> ValueError: <function myHandler at 0x1146b4c80>: returned None, expecting a value

Looking a the metadata the error message is wrong, the handler should not return a value (retval for the last argument is ‘v’):

:>>> pprint.pprint(Cocoa.NSEvent.addLocalMonitorForEventsMatchingMask_handler_.__metadata__())
{'arguments': ({'_template': True, 'type': b'@'},
               {'_template': True, 'type': b':'},
               {'_template': True, 'type': b'Q'},
               {'callable': {'arguments': ({'null_accepted': True,
                                            'type': b'^v'},
                                           {'type': b'@'}),
                             'retval': {'type': b'v'}},
                'callable_retained': True,
                'type': b'@?'}),
 'classmethod': True,
 'hidden': False,
 'retval': {'_template': True, 'type': b'@'}}


However…. that is a second bug, the return value should be an NSEvent value. That is, your code is correct, but PyObjC contains two bugs: 1) the metadata incorrectly says that the handler should not return a value, and 2) the error message when returning a value from a block where no return value is expected is wrong.

I’ve committed a fix for the second problem, and will shortly commit a fix for the incorrect metadata as well.

A quick workaround: Look for “addLocalMonitorForEventsMatchingMask:handler:” in Lib/AppKit/_metadata.py and replace ‘retval’: ‘v’ by ‘retval’: ‘@‘.

Ronald