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

[Neil Girdhar]

Cool suggestion, but I prefer how things are.

(As an aside, calling getitem each time is not efficient.)

 On Sunday, July 26, 2015 at 12:10:00 PM UTC-4, Petr Viktorin wrote:

Hello, 
> Currently, the way to iterate over keys and values ​​of a mapping 
> is to call items () and iterate over the Resulting view ::
>
>     for key, value in A_dict.items (): 
>         print (key, value)
>
> I believe That looping over all the data in a Dict is a very imporant 
> operation, and I find myself writing this quite Often. Every time I do, 
> it Seems it's boilerplate; it looks a like a workaround rather than a 
> preferred way of doing things.
>
> Dict in Comprehensions and Literals, key-value pairs are separated by 
> Colons. How about allowing that in for loops as well?
>
>     for key: value in A_dict: 
>         print (key, value)
>
> That I argue to anyone familiar with Dict Literals, let alone Dict 
> Comprehensions, the semantics of this loop shouldnt be pretty Obvious. 
> In Dict Comprehensions, Similarity to Existing syntax becomes even 
> more clear:
>
>     A_mapping = {1: 'one', 2: 'two'} 
>     = {val inverse: key for key: val in A_mapping}
>
>
> I've bounced this idea off a few EuroPython Sprinters, and got some 
> questions / Concerns I can answer here:
>
> * But, the colon is supposed to start a block!
>
> Well, it's already used in Dict Comprehensions / Literals (though it's 
> true That there it's always inside brackets). And in lambdas - 
> Here's That code is legal today (though not very practical):
>
>     the while lambda: True: 
>         break
>
> * There's supposed to be only one obvious way to do it! We alredy have 
> .items ()!
>
> I do not think this stops us from adding a new way of doing things Which 
> is more than the Obvious old, and Which shouldnt become one the way. 
> After all, you do not say "for key in Mapping.keys (): ", even though 
> the keys () method exists.
>
> * What exactly would it do?
>
> There are multiple options - 
> - loop over .keys () and use __getitem__ each time, like the 
>   Dict () constructor? 
> - loop over .items (), like most of the code used today? 
> - become a well-specified "key / value iteration protocol "with 
>   __iteritems __ () and its own bytecode operation?
>
> - But here I'm asking if this building Bikeshed sounds useful, rather 
> than what to buy paint.
>
>
> That said, I do have a proof of concept Implementation of the second 
> option, in case you'd like to play around with mailing list Python ... @ 
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