pyobjc does not throw an error and instead returns None

[yasoob]

pyobjc does not throw an error and instead returns None when creating something with the wrong arguments. For instance, one proper way to create a CIImage is this:

import Quartz
from Foundation import NSURL

input_url = NSURL.fileURLWithPath_(img_path)
input_image = Quartz.CIImage.imageWithContentsOfURL_(input_url)

But if I pass in a string to imageWithContentsOfURL_ method instead of an NSURL, it doesn't throw an error. It simply returns None.

I am very new to pyobjc and even Objective C. I am trying to figure out if I am doing something wrong? Is there any way to make pyobjc throw an error instead of returning None? Or maybe returning None is how it works in Objective C too.

I even tried setting the verbose to True:

import objc
objc.options.verbose = True

But it didn't improve things. Can you kindly guide what I might be doing wrong? It is very hard to debug bugs without any indication that what I am doing is wrong.

[ronaldoussoren]

In this particular case the equivalent incorrect Objective-C code returns nil which maps to None in Python.

#import <CoreImage/CoreImage.h>
#import <Foundation/Foundation.h>

int main(void)
{
    NSLog(@"%@", [CIImage imageWithContentsOfURL:(NSURL*)@"path"]);
}

Sadly the Cocoa frameworks contain limited error checking, mostly relying on the compiler to find problems like this. If Objective-C code raises an exception that exception is automatically translated to a Python exception. The verbose option is primarily present to log exceptions that are later swallowed by a runloop to make it easier to debug code.

There will be type annotations for PyObjC in the future, but adding those is a fair bit of work and may require some work with the type community as well because PyObjC does some things that don't necessarily play nice with type checkers.

For now I generally keep Apple's documentation open when working with APIs that I haven't used enough.

[yasoob]

Ah ok this makes sense. Thanks for the quick response. I will close this issue then. Either ways, thanks for all your work! I worked with pyobjc for the first time recently and it is pretty amazing to see what it is capable of :)