On Thu, Feb 21, 2019 at 10:04 PM Steven D'Aprano <st...@pearwood.info> wrote: > > On Thu, Feb 21, 2019 at 07:05:55PM +1100, Chris Angelico wrote: > > [Ben] > > > Other functions also conceptually have three ways of returning: > > > ordinary return with a value, a documented special return like > > > KeyError, and pass-through exceptions. If the pass-through exception > > > is KeyError, it gets conflated with the documented exceptional return, > > > but correct code should handle them differently. It doesn't matter > > > whether the syntax for the documented special return is "return x" or > > > "raise KeyError(x)". > > > > Not sure what you mean here. If the documented special return is > > "return x", then it's a value that's being returned. That's completely > > different from raising some sort of exception. > > I think I understand what Ben means, because I think I've experimented > with functions which do what he may be getting at. > > Remember that exceptions are not necessarily errors. So you might write > a function which returns a value in the standard case, but raises an > exception to represent an exceptional case. If this is not an error, > then the caller ought to be prepared for the exception and always catch > it.
Yep, I understand that part (and mention KeyError specifically). What I don't understand is the documented special return of "return x". If there's a way to return a magic value, then it's still just a value. For the rest, yeah, there's the normal Python behaviour of signalling "nope" by raising a specific exception. And yes, leaking an exception of the same type from within that function is going to be interpreted as that "nope". That's important to the ability to refactor - you can have a helper function that raises, and then the main __getitem__ or __getattr__ or whatever will just propagate the exception. That's why "leaking" is such a hard thing to pin down. ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
