Re: [Pyobjc-dev] the mysterious attribute error

[Bob I. <bo...@re...>]

On Mar 28, 2004, at 2:26 PM, Ronald Oussoren wrote:

> On 28-mrt-04, at 18:36, Burris T. Ewell wrote:
>
>> Another clue:
>>
>> *** malloc[5538]: error for object 0x5b9f00: Double free
>>
>> Somewhere, memory management is going awry.
>
> That's consistent with my diagnosis. I'm thinking about a fix for 
> this, but I have not yet found a good solution for this.
>
> The problem is that __dict__ and attributes introduced by __slots__ 
> are 'PyObject*' attributes when you look at the object from 
> Objective-C. These are copied by the default copyWithZone: but without 
> updating the reference count (Py_INCREF-ing the value). That means 
> we're hosed when copy/copyWithZone: is used, because it is not 
> possible to fix the reference count using Python code.
>
> It is easy enough to silently introduce a copyWithZone: method that 
> does the right thing, but that won't work when someone tries to 
> implement a copyWithZone: method in Python (e.g. for customizing the 
> copy).
>
> When someone adds 'def copyWithZone_(self, zone)' to a new class, we 
> cannot insert the generic PyObjC-copyWithZone: implementation without 
> either adding serious hacks, e.g. looking for calls to the super-class 
> implementation of copyWithZone: and replacing them by calls to a 
> helper function, or changing the semantics.
>
> The best option seems to be to change the interface for copyWithZone_: 
> if you implement copyWithZone_ in Python it must have the following 
> interface:
>
> 	def copyWithZone_(self, zone, copy):
> 		# Update copy here
> 		return copy
>
>
> I don't really like this, but don't see how we can have the normal 
> semantics and yet be able to insert our own method into the 
> call-chain. The reason I don't like this is that this interface 
> can/should only be used when subclassing a class that implements 
> NSCopying.
>
> Luckily we can give a usefull error-message when someone does 
> implement copyWithZone_ but doesn't use the right inteface.

Why not just force a default copy and copyWithZone implementation (as 
far as Python is concerned), and instead use the __getstate__ and/or 
__reduce__ method of copying python things?

-bob