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

Sven R. Kunze srkunze at mail.de
Sun Jul 26 22:05:02 CEST 2015

I'd love that because I find .items() quite cumbersome as well if I have 
to use it.

I'd like to know if there were some reason not to introduce this in the 
first place.


On 26.07.2015 18:09, 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.
> In dict 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)
> I argue that to anyone familiar with dict literals, let alone dict
> comprehensions, the semantics of this loop should be pretty obvious.
> In dict comprehensions, similarity to existing syntax becomes even
> more clear:
>      a_mapping = {1: 'one', 2: 'two'}
>      inverse = {val: 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 code that is legal today (though not very practical):
>      while lambda: True:
>          break
> * There's supposed to be only one obvious way to do it! We alredy have .items()!
> I don't think this stops us from adding a new way of doing things which
> is more obvious than the old, and which should become the one way.
> After all, you don't 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 building this bikeshed sounds useful, rather
> than what paint to buy.
> That said, I do have a proof of concept implementation of the second
> option, in case you'd like to play around with this:
> Github: https://github.com/encukou/cpython/tree/keyval-iteration
> patch: https://github.com/encukou/cpython/commit/b9b0d973342280f0ef52e26a4b67f326ece82a54.patch
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