On 7 April 2011 15:36, Nick Coghlan <ncoghlan@gmail.com> wrote:
On Thu, Apr 7, 2011 at 11:48 PM, Michael Foord <fuzzyman@gmail.com> wrote:
Hmmm... that would rely on subclass.__new__ both existing *and* not calling up to its parent __new__ or you will have infinite recursion. Probably what you have to do is call object.__new__(subclass, ...) and knowing / praying that subclass.__new__ doesn't do anything important...
That's the dance I'm trying to remember. You make it work by playing identity checking games with the cls argument, but it's been absolutely ages since I read about it and experimented with it.
I think it's something like:
def __new__(cls, *args, **kwds): if cls is ThisClass: # Do fancy footwork to implicitly create an appropriate subclass instead # via subclass.__new__ obj = cls._create_instance(*args, **kwds) else: # Don't do anything tricky for subclasses, that's their problem obj = object.__new__(*args, **kwds) return obj
Subclasses then have the option of passing the parent class as "cls" if they want to invoke the fancy footwork, or themselves if they don't.
Nice solution. You should write it up on your blog. It lets you call subclass.__new__, to return instances of subclasses, without having to worry about whether or not subclass.__new__ is going to upcall. Michael
Cheers, Nick.
-- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia
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