On 7 April 2011 12:48, Nick Coghlan <ncoghlan@gmail.com> wrote:

On Thu, Apr 7, 2011 at 7:17 PM, Antoine Pitrou <solipsis@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__(...) 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@gmail.com   |   Brisbane, Australia

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