On Fri, Mar 27, 2009 at 12:53 AM, Greg Ewing <greg.ew...@canterbury.ac.nz> wrote: > Guido van Rossum wrote: > >> That +0 could turn into a +1 if there was a way to flag this as an >> error (at runtime), at least if the return is actually executed: >> >> def g(): >> yield 42 >> return 43 >> >> for x in g(): >> print x # probably expected to print 42 and then 43 >> >> Perhaps the exception used in this case could be a different exception >> than StopIteration? > > Would checking that the value is None be sufficient? Or do you > want to distinguish between 'return' and 'return None'? That > would feel rather strange to me.
Well currently there *is* a difference, because "return None" is a (phase 2) SyntaxError in a generator, while "return" isn't. But I don't really care if you cannot distinguish these two cases. > I'm inclined to regard this as an unnecessary complication. > People are already trained to think of 'return' and 'yield' > as quite different things. I don't know why they would suddenly > start using 'return' when they mean 'yield'. Because coroutines are mind-bendingly hard, and it's easy to confuse yield (pass a value back while keeping the stack frame) and return (pass a value back dropping the stack frame). The syntactic prohibition on "return <expr>" in generators currently helps people to be consistent. I'd like to keep this small crutch around so that people who are writing vanilla (== meant for iteration) generators are kept from accidentally introducing hard-to-debug problems. Perhaps the crux is that *if* you accidentally use "return <value>" in a vanilla generator expecting the value to show up somewhere, you are probably enough of a newbie that debugging this will be quite hard. I'd like not to have such a newbie trap lying around. -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ 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/archive%40mail-archive.com
