pyobjc-dev

[Pyobjc-dev] Returned BOOL values from Objc to Python

[Patrick R. <rob...@gm...>]

Hi all,

I'm currently creating a PyObjc app which uses a mixture of PyObjc and Plain Objc (mainly for view stuff).
I have a method in one of my (Objective-C) view controllers as follows:

@implementation MyVC
- (BOOL)isVisible {
	return [self.mySpecialView isVisible]
}

in Python, I call this:

# myviewcontroller =  MyVC.alloc().init()
# …stuff

isVisible = myviewcontroller.isVisible()
if isVisible is False:
	# do something special

Now, I'd expect this call through to my Objective-C method to return a Python bool value of True or False. However, it seems to return an int (1 or 0)
I've checked this with repr(isVisible) and what have you.
This subsequently causes the line:

	if isVisible is False:

to fail (since 0 is not False)
Of course I can do

	if isVisible == False:

to get round this, but I feel like if I'm declaring a BOOL in Obj-C I should get one pack in Python

P.S. I've tried casting the return value of my Obj-C method to a BOOL just incase that'd make a difference. It doesn't.

Thanks for any help

Re: [Pyobjc-dev] Returned BOOL values from Objc to Python

[Wool S. S. <woo...@gm...>]

> isVisible = myviewcontroller.isVisible()
> if isVisible is False:
> # do something special

The problem is that you shouldn't use 

   if x is False:

to test falsehood. Nor, according to PEP 8, should you use  

   if x == False:

Instead, use

   if not x:

Python's is operator compares object identity, not value. Your test will fail for any "false-y" value that's not the False singleton itself, like None or an empty list.

> it seems to return an int (1 or 0)

In case you're not aware of it, ObjC's BOOL is not a true boolean type; it's just a typedef'd signed char, so the integer 0 or 1 you get is strictly correct. There's actually no way for PyObjC to know that it should be a turned into a Python boolean, unless you create bridge metadata for your method.

-- Josh Caswell