[Python-Dev] for...else (original) (raw)

Ben Hoyt benhoyt at gmail.com
Mon Jul 24 12:23:49 EDT 2017

This is more of a python-ideas discussion, and Steven's answer is good.

I'll just add one thing. Maybe it's obvious to others, but I've liked for...else since I found a kind of mnemonic to help me remember when the "else" part happens: I think of it not as "for ... else" but as "break ... else" -- saying it this way makes it clear to me that the break goes with the else. "If this condition inside the loop is true, break. ... else if we didn't break, do this other thing after the loop."

-Ben

On Mon, Jul 24, 2017 at 12:14 PM, Steven D'Aprano <steve at pearwood.info> wrote:

Hello Kiuhnm, and welcome.

On Mon, Jul 24, 2017 at 05:35:03PM +0200, Kiuhnm via Python-Dev wrote: > Hello, > > I think that the expression "for...else" or "while...else" is completely > counter-intuitive.

You may be right -- this has been discussed many, many times before. In my personal opinion, the best (and only accurate!) phrase would have been: for item in sequence: # block then: # block If you look at the byte-code generated by a for...else statement, you see that the "else" block is unconditionally executed after the for loop completes, unless something causes a jump outside of the entire statement: return, break, or raise. So it is more like: - run the loop; - then run the following block rather than: - run the loop; - otherwise ("else") run the following block. Others disagree and would prefer other keywords. But regardless, backwards compatibility means that we must keep "for...else", so I'm afraid that discussing alternatives is almost certainly a waste of time. > Wouldn't it be possible to make it clearer? Maybe > something like At this point, no, it is not practical to change the syntax used. Maybe when Python 3.0 was first introduced, but that ship has long sailed. It is very, very unlikely that the syntax for this will ever change, but if it does, it probably won't be until something in the distant future like Python 5. But not Python 4: Guido has already ruled that Python 4 will not include major backwards-incompatible changes. Going from 3 to 4 will not be as disruptive as going from 2 to 3. So depending on how you look at it: discussing alternative syntax to for...else is either ten years too late or ten years too early.

-- Steve

Python-Dev mailing list Python-Dev at python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/ benhoyt%40gmail.com -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20170724/89770549/attachment.html>

More information about the Python-Dev mailing list