[Python-ideas] PEP-3151 pattern-matching

[Michael Foord]

On 7 April 2011 14:32, Michael Foord <fuzzyman at gmail.com> wrote:

>
>
> On 7 April 2011 12:48, Nick Coghlan <ncoghlan at gmail.com> wrote:
>
>> On Thu, Apr 7, 2011 at 7:17 PM, Antoine Pitrou <solipsis at pitrou.net>
>> wrote:
>> >> Perhaps the IOError constructor could be made to switch
>> >> the class of the generated object based on the errno
>> >> attribute passed to the constructor.
>> >
>> > Nice suggestion. I'll try to see if that is possible.
>>
>> It should be possible with appropriate fancy footwork in __new__. You
>> do need to be careful to avoid calling __init__ on the created object
>> twice.
>>
>> I know I've read an example that demonstrates the principle, but I
>> unfortunately don't remember where (I initially thought it was in
>> Guido's new-style class essay, but I checked and that wasn't it)
>>
>
>
> __init__ is called for you on construction so long as the object returned
> by __new__ is an instance of the type being constructed or a subclass.
>
> So if what you want __new__ to return *is* a subclass, then you create it
> with subclass.__new__(...)
>

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...

All the best,

Michael Foord


> and not subclass(...) (the latter would call __init__ which would then be
> called again after __new__ returns).
>
> If what you're returning *isn't* a subclass (which is best avoided anyway)
> then you can construct it with otherclass(...) as __init__ won't be called
> for you.
>
> All the best,
>
> Michael Foord
>
>
>>
>> Cheers,
>> Nick.
>>
>> --
>> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>> _______________________________________________
>> Python-ideas mailing list
>> Python-ideas at python.org
>> http://mail.python.org/mailman/listinfo/python-ideas
>>
>
>
>
> --
>
> http://www.voidspace.org.uk/
>
> May you do good and not evil
> May you find forgiveness for yourself and forgive others
>
> May you share freely, never taking more than you give.
> -- the sqlite blessing http://www.sqlite.org/different.html
>
>
>


-- 

http://www.voidspace.org.uk/

May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing http://www.sqlite.org/different.html
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