On 7 April 2011 14:32, Michael Foord <fuzzyman@gmail.com> wrote:
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.
Those are the pure python rules anyway (which you were probably aware of), no idea if it is different in C. :-) Michael
All the best,
Michael Foord
Cheers, Nick.
-- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas
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-- 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