[Python-ideas] PEP-3151 pattern-matching

[Michael Foord]

On 7 April 2011 15:36, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On Thu, Apr 7, 2011 at 11:48 PM, Michael Foord <fuzzyman at 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 at gmail.com   |   Brisbane, Australia
>



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