On Wed, 29 Sep 2010 14:34:33 +0200, Hrvoje Niksic wrote: > Steven D'Aprano <st...@remove-this-cybersource.com.au> writes: >> (This may change in the future. Given type(), and isinstance(), I'm not >> sure what value __class__ adds.) > > None whatsoever. __class__ used to be necessary to tell the appart > instances of different old-style classes: > >>>> class X: pass # old-style > ... >>>> X().__class__ > <class __main__.X at 0xb772f2fc> >>>> type(X()) > <type 'instance'> > > Now that classes produce real types, they are equivalent.
I've been thinking about this, and I think that there may be a use-case for changing __class__ manually. I'm not entirely sure if this is a good idea or not, I'd need to spend some time experimenting, but imagine a class that wraps another object and uses delegation instead of inheritance. If you also set __class__ (on the class itself, naturally, not the instance) appropriately, then type(instance) and instance.__class__ will be different. type() will return the "real" type of the delegation class, while instance.__class__ returns the class that it is trying to be. And of course, presumably metaclasses can do anything they like with __class__, for good or ill. I'm not sure if and when this would be useful, but it's a hint that perhaps the distinction is not entirely meaningless. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
