problems with __getattr__ & __setattr__

[ScherBi at BAM.com]

Michael Hudson wrote:

>      def __getattr__(self, key):
>          try:
>              return self.__dict__["eggs"][key]
>          except:
>              raise
> 
> is probably safer. What are the try/catch for? They aren't doing
> anything here!

Thanks.  I came upon your this solution in the interval.
The try/except were not relevent in the sample, but are in my real code.
sorry. 8-)

> __getattr__-and-__setattr__-are-so-much-fun-we-don't-mind-the-
> performance-hit-ly y'rs - Michael

Yes, I had to convince a colleague (it wasn't hard) that the syntactic sugar
they produce are worth the __dict__ mangling ugliness. :)

Thanks again.

Bill



> -----Original Message-----
> From: Michael Hudson [mailto:mwh21 at cam.ac.uk]
> Sent: Tuesday, November 16, 1999 3:41 PM
> To: python-list at python.org
> Subject: Re: problems with __getattr__ & __setattr__
> 
> 
> ScherBi at BAM.com writes:
> 
> > I am having trouble using __getattr__ and __setattr__ in 
> some classes.  I've
> > boiled it down to the code below.
> > It dumps core under 1.5.2 on Linux or WinNT.
> > 
> > I think maybe it's looping, if so, how does one go about 
> doing this? 
> > (I'm assuming it's clear enough what I'm trying to do.)
> > 
> > If I comment out the __gettattr__ operation, it doesn't 
> dump core.  Instead
> > I get an AttributeError: eggs raised at the 'self.eggs[key] 
> = value' call in
> > __setattr__.  This is what leads me to belive there's 
> something circular
> > going on here.
> > 
> > 
> > Thanks,
> >  Bill
> > 
> > -------------------------------------------------------------
> > 
> > class spam:
> > 
> >     def __init__(self):
> >         self.eggs = {}
> 
> Hint: what does this call?
>      
> >     def __getattr__(self, key):
> >         try:
> >             return self.eggs[key]
> >         except:
> >             raise
> >         
> >     def __setattr__(self, key, value):
> 
> It calls this!
> 
> >         try:
> >             self.eggs[key] = value
> 
> And this calls __getattr__ which looks for self.eggs, which calls
> __getattr__, which looks for self.eggs, which calls __getattr__, which
> looks for self.eggs, which calls __getattr__, which looks for
> self.eggs, which calls __getattr__, which looks for self.eggs, which
> calls __getattr__, 
> Segmentation fault (core dumped)
> 
> >             return 0
> >         except:
> >             raise
> > 
> > 
> > 
> > if __name__ == '__main__':
> > 
> >     s = spam()
> >     print 'instance of spam created.' 
> > 
> 
> try
> 
>     def __init__(self):
>         self.__dict__['eggs'] = {}
> 
> instead; also 
> 
>      def __getattr__(self, key):
>          try:
>              return self.__dict__["eggs"][key]
>          except:
>              raise
> 
> is probably safer. What are the try/catch for? They aren't doing
> anything here!
> 
> __getattr__-and-__setattr__-are-so-much-fun-we-don't-mind-the-
> performance-hit-ly y'rs - Michael
> 
> -- 
> http://www.python.org/mailman/listinfo/python-list
>