[Python-Dev] buitlins instance have modifiable __class__?

[Guido van Rossum]

> question on bultin types (under 2.2):
> 
> >>> d={}
> >>> class ndict(dict):
> ...   __slots__ = ()
> ...   def __getitem__(self,k):
> ...    print "__getitem__"
> ...    return dict.__getitem__(self,k)
> ...
> >>> d.items()
> []
> >>> d['a']=3
> >>> d.__class__=ndict
> 
> is intended to work?
> 
> it seems it does, but is that the intention?

It was a mistake.  In 2.3, it's disallowed.  In 2.2.2, it'll still be
allowed, but you shouldn't do this -- all sorts of bizarre stuff can
happen because you can do this.

So if you're asking about this for Jython, please don't allow this in
Jython!

> typos apart, there was also another question, sorry I was typing and
> reflecting on the consequences of all of this on Jython ...
> 
> [me]
> >
> > >>> exec "print a" in d
> > 3
> >
> > Ok, that is the non cooperative behavior I already know about. ]
> >
> 
> I recall this was already discussed here, what is the idea, to leave
> it as it is or make this work?

That's not going to change in CPython, because I believe it would slow
down lookup for builtins and globals too much if we had to check for a
custom __getitem__.  But if you can fix it for Jython, go ahead.  I
don't mind if there are places where Jython is "purer" than CPython.

--Guido van Rossum (home page: http://www.python.org/~guido/)