[Pyobjc-dev] Fwd: TypeError: cannot change a method

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

Begin forwarded message:

> From: Ronald Oussoren <ron...@ma...>
> Date: 17 July, 2009 6:53:01 GMT+02:00
> To: Daniel Miller <mil...@gm...>
> Subject: Re: [Pyobjc-dev] TypeError: cannot change a method
>
>
> On 17 Jul, 2009, at 3:09, Daniel Miller wrote:
>>>
>>
>> I'm not quite sure what to say other than I'll need to look into  
>> rewriting my test framework. FWIW, it seems to be possible to  
>> replace a method directly on the PyObjC type. For example:
>>
>> # this works
>> fake = descriptor_mock()
>> NSDocument.removeWindowController_ = fake
>>
>> Assuming the 'fake' object implements the descriptor protocol, the  
>> fake/mocked method will get called as expected.
>> But it then it fails when trying to restore the original method:
>>
>> NSDocument.removeWindowController_ = original
>> # TypeError: Assigning native selectors is not supported
>>
>> However, this seems to restore the original:
>>
>> del NSDocument.removeWindowController_
>>
>> Strange... while I may be able to use this approach, it has the  
>> disadvantage of replacing the method on ALL instances of the type  
>> rather than on just a the instance that I'm testing--I may need to  
>> rewrite some tests to work with that restriction, but it probably   
>> wouldn't be the end of the world.
>
> You're not going to like this, but the behaviour you describe here  
> sounds like a bug. In particular, it is possible to overwrite an  
> existing method using a new function object but it should not be  
> possible to overwrite it with something else. Removing the fake  
> descriptor should also not result in revival of the original method  
> because a replacement method should result in a changed method in  
> the ObjC runtime.
>
> I need to think about your testing needs, if it is possible to  
> support those and if so how.
>
> A number of limititations in PyObjC, such as this one, are caused by  
> the need to interact with the Objective-C runtime. In particular  
> "objc.selector" objects for native methods are basicly references to  
> a named selector on the ObjC class, they do not contain a reference  
> to the code that will be called when you call the method. That's why  
> it is not possible to assign objc.selector instances to a class  
> attribute.
>
> BTW. How does your mocking library deal with other native types:
>
> >>> l = list()
> >>> l.append = 42
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> AttributeError: 'list' object attribute 'append' is read-only
> >>>
>
> Or more interesting, how does it deal with special methods like  
> __getitem__, those always get resolved through the class:
>
> >>> class List (list): pass
> ...
> >>> l = List()
> >>> l.append(42)
> >>> l[0]
> 42
> >>> l.__getitem__ = 42
> >>> l[0]
> 42
> >>>
>
> As you can see setting __getitem__ on an instance has no effect on  
> which special method is actually used.
>
> Ronald
>>
>> ~ Daniel
>>
>