On 5 Apr 2013 01:07, "Chris Angelico" <ros...@gmail.com> wrote: > > On Fri, Apr 5, 2013 at 1:59 AM, Guido van Rossum <gu...@python.org> wrote: > > On Thu, Apr 4, 2013 at 7:47 AM, Chris Angelico <ros...@gmail.com> wrote: > >> Is there any argument that I can pass to Foo() to get back a Bar()? > >> Would anyone expect there to be one? Sure, I could override __new__ to > >> do stupid things, but in terms of logical expectations, I'd expect > >> that Foo(x) will return a Foo object, not a Bar object. Why should int > >> be any different? What have I missed here? > > > > > > A class can define a __new__ method that returns a different object. E.g. > > (python 3): > > > > Right, I'm aware it's possible. But who would expect it of a class?
Python 3.3 does it for OSError to map errno values to the appropriate subclasses. That's mainly to aid migration to the new exception structure, though (see PEP 3151). For a clean slate API design you would use a separate factory function or class method to do the conversion. Cheers, Nick. > > ChrisA > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > http://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: http://mail.python.org/mailman/options/python-dev/ncoghlan%40gmail.com
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