getattr and setattr

[Dale Strickland-Clark]

Very useful. Thanks.

--
Dale Strickland-Clark
Out-Think Ltd, UK
Business Technology Consultants

Thomas Wouters <thomas at xs4all.net> wrote in message
news:20000706155320.A25064 at xs4all.nl...
> On Thu, Jul 06, 2000 at 01:58:45PM +0100, Dale Strickland-Clark wrote:
>
> > I'm still trying to get to grips with object programming in Python and
> > finding decent documentation unbelievably hard to track down.
>
> Hm, I've never needed much more than the documentation on www.python.org:
> the tutorial at first, then the language ref and the library ref ;)
> The only parts lacking in there are Tkinter, and some modules which aren't
> documented yet.
>
> > I eagerly set about the tutorial but found it was  lacking in - well -
just
> > about every department.
>
> The tutorial is more of an intro than a comprehensive educational track.
Try
> the Language Reference or the Library Reference. If you don't care for the
> dull reference-style writing, try something like Learning Python, by
> O'Reilly, or one of the other online tutorials available (sorry, no URLs
;P)
>
> > I can't find any meat on __getattr__ and __setattr__.
> > These two special object functions don't seem to be very well documented
> > anywhere. (Please prove me wrong!)
>
> The language reference, section 3.3, 'Special Method Names':
> http://www.python.org/doc/ref/specialnames.html. To be more specific,
> section 3.3.2, 'Customizing attribute access'.
>
> > I can see how to handle a special case in __getattr__ with something
like:
>
> > class wibble
> >         def __getattr__(name):
> >                 if name == 'fred':
> >                         return 1
> >                 else:
> >                         # what?
>
> > But what if it's not the special case(s). How do I return an arbitrary
> > attribute? Do I do something like:
>
> __getattr__ is only called if the attribute does *not* exist on the
object.
> The reason is fairly simple: if it was called for every lookup,
__getattr__
> itself could never be looked up !
>
> If you decide, in your __getatr__, that an attribute shouldn't exist, you
> should raise AttributeError. Don't do something like 'return 0' or you'll
> confuse all kinds of internal mechanisms, like those that lookup special
> method names ;)
>
> __setattr__, however, is called for every attempt at storing a value in an
> attribute, wether it exists or not. If you use __setattr__ and want to
> alter an attribute, you shouldn't do the obvious ! If you assign from
inside
> __setattr__, __setattr__ is called again. Instead, you should modify
> self.__dict__.
>
> Section 3.3.2 explains all this, but in a bit less (and different ;)
words.
> --
> Thomas Wouters <thomas at xs4all.net>
>
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>