[Python-ideas] Syntax for key-value iteration over mappings

[Steven D'Aprano]

On Mon, Jul 27, 2015 at 05:30:38PM +0200, Sven R. Kunze wrote:
> On 27.07.2015 13:42, Lennart Regebro wrote:
> >On Mon, Jul 27, 2015 at 4:12 AM, Steven D'Aprano <steve at pearwood.info> 
> >wrote:
> >>It's one more special case syntax for beginners to learn. And it really
> >>is a special case: there's nothing about "for k:v in iterable" that
> >>tells you that iterable must have an items() method. You have to
> >>memorise that fact.
> >This I think is a strong argument.
> 
> I cannot follow. There is nothing about 'await' that tells me it can 
> only be used with coroutines. I need to memorize that fact, too.

Yes, and you need to memorise what "for" loops do, and "len()", etc. But 
if you know English, the name is an aid to memory. There's no aid to 
memory with a:b syntax, and googling for it will be a pain. 

Not everything is important enough to be given its own syntax. That way 
leads past Perl and into APL. (At least APL tries to follow standard 
mathematical notation, rather than being a collection of arbitrary 
symbols.)

`await` gives us a whole lot of new functionality that was hard or 
impossible to do before. What does this give us that we couldn't do 
before? What's so special about spam.items() that it needs dedicated 
syntax for it?

These are not rhetorical questions. If you can answer those positively, 
then I'll reconsider my opposition to this. But if the only thing this 
syntax gains us is to avoid an explicit call to .items(), then it just 
adds unnecessary cruft to the language.


> >What error would you get when it's the wrong type? An attribute error
> >on .items(), or a special SyntaxError "This syntax can only be used on
> >mappings".
> 
> I would like such an error. Because it tells me that it is not what I 
> wanted. The current methods silently works and I get an error later.

I don't understand you. If you write `for k,v in spam.items()` and spam 
has no items method, you get an AttributeError immediately. How does it 
silently work?



-- 
Steve