Re: [Pyobjc-dev] "out" parameters and return values

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

On 19 Feb, 2009, at 5:16, Mani Ghasemlou wrote:

> Thanks so much. Then, there must be something wrong with my code,
> because I keep getting a crash when hitting this code. This however
> rules out that my overall approach is wrong.

Can you reproduce your crash in a smaller program? That would allow us  
to look at the code and could help pintpoint the issue (either  
something in your code or a bug in PyObjC).

Is the method your writing overriding an existing method, or  
implementing an informal protocol?  PyObjC cannot deduce the right  
signature for methods with output arguments without that information.  
An easy way to check if that's your problem is to add some   
decoration[1] to your code that explicitly sets the signature:

    def returnIntWithError_(self, outError):
         pass # body goes here
    returnIntWithError_ = objc.selector(returnIntWithError_,  
signature='@@:o^@')

Ronald

[1] the reason for this strange formulation is that I wanted to write  
"a decorator", but noticed that the required decorator isn't actually  
part of PyObjC yet.

>
>
> Kind regards,
> Mani
>
> On Wed, Feb 18, 2009 at 4:17 PM, Ronald Oussoren <ron...@ma... 
> > wrote:
>> Hi,
>>
>> You're approach is correct.
>>
>>>> def returnIntWithError_(self, outError):
>>>> myError = NSError.errorWithDomain_code_userInfo_('myDomain', '1',
>>>> objc.nil)
>>>> myReturnVal = 1
>>>> return myReturnVal, myError
>>>
>>
>> As an optimization you could check if "outError is objc.NULL", if  
>> it is the
>> caller isn't interested in the error value and you could return
>> (myReturnVal, objc.NULL) instead of (myReturnVal, myError).    
>> That's not
>> really an issue here, but could help when it is hard to calculate  
>> the value
>> of the output argument.
>>
>> BTW. Use "None" instead of "objc.nil", both have the same value and
>> "objc.nil" is basicly only present to make it easier to translate  
>> ObjC code
>> into Python. The same is true for objc.YES and objc.NO, those are  
>> aliases
>> for True and False.
>>
>> Ronald
>>
>>
>> On 17 Feb, 2009, at 20:51, Mani Ghasemlou wrote:
>>
>>> Just wanted to raise this question again, as I still haven't found a
>>> solution.
>>>
>>> I've posted the same question on Cocoa-dev, also with no response:
>>> http://www.cocoabuilder.com/archive/message/cocoa/2009/2/2/229316
>>>
>>> Would greatly appreciate any insights!
>>>
>>> Kind regards,
>>> Mani
>>>
>>> On Mon, Feb 2, 2009 at 9:34 AM, Mani Ghasemlou <ma...@tu...>  
>>> wrote:
>>>>
>>>> Hi all,
>>>>
>>>> From what I understand of the PyObjc documentation, "out" pointer
>>>> variables (such as "NSError **outError") are actually appended to  
>>>> the
>>>> list of return values for a bridge function.
>>>>
>>>> For example:
>>>>
>>>> int returnIntWithError: NSError **outError
>>>>
>>>> from Python would be invoked as:
>>>>
>>>> returned_int, returned_error = returnIntWithError_(None)
>>>>
>>>> I hope that my understanding of the above is correct.
>>>>
>>>> Now, my real question is to extend the above concept to the Python
>>>> side, when we want to implement an informal protocol.  Let's say  
>>>> the
>>>> protocol says that I have to implement the returnIntWithError:
>>>> function as described above. How do I implement this function? Is  
>>>> the
>>>> code below enough?
>>>>
>>>> def returnIntWithError_(self, outError):
>>>> myError = NSError.errorWithDomain_code_userInfo_('myDomain', '1',
>>>> objc.nil)
>>>> myReturnVal = 1
>>>> return myReturnVal, myError
>>>>
>>>> Note that I am completely ignoring the "outError" parameter. Does  
>>>> this
>>>> look kosher?
>>>>
>>>> Thanks in advance for any help!
>>>>
>>>> Cheers,
>>>> Mani
>>>>
>>>
>>>
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>>