Python 3: dict & dict.keys()

Oscar Benjamin oscar.j.benjamin at gmail.com
Wed Jul 24 18:32:01 CEST 2013


On Jul 24, 2013 2:27 PM, "Peter Otten" <__peter__ at web.de> wrote:
>
> Oscar Benjamin wrote:
>
> > On Jul 24, 2013 7:25 AM, "Peter Otten" <__peter__ at web.de> wrote:
> >>
> >> Ethan Furman wrote:
> >>
> >> > So, my question boils down to:  in Python 3 how is dict.keys()
> >> > different
> >> > from dict?  What are the use cases?
> >>
> >> I just grepped through /usr/lib/python3, and could not identify a
single
> >> line where some_object.keys() wasn't either wrapped in a list (or set,
> >> sorted, max) call, or iterated over.
> >>
> >> To me it looks like views are a solution waiting for a problem.
> >
> > What do you mean? Why would you want to create a temporary list just to
> > iterate over it explicitly or implicitly (set, sorted, max,...)?
>
> I mean I don't understand the necessity of views when all actual usecases
> need iterators. The 2.x iterkeys()/iteritems()/itervalues() methods didn't
> create lists either.

Oh, okay. I see what you mean.

>
> Do you have 2.x code lying around where you get a significant advantage by
> picking some_dict.viewkeys() over some_dict.iterkeys()?

No. I don't think I've ever used viewkeys. I noticed it once, didn't see an
immediate use and forgot about it but...

> I could construct
> one
>
> >>> d = dict(a=1, b=2, c=3)
> >>> e = dict(b=4, c=5, d=6)
> >>> d.viewkeys() & e.viewkeys()
> set(['c', 'b'])

that might be useful.

>
> but have not seen it in the wild.

> My guess is that most non-hardcore users don't even know about viewkeys().
> By the way, my favourite idiom to iterate over the keys in both Python 2
and
> 3 is -- for example -- max(some_dict) rather than
> max(some_dict.whateverkeys()).

Agreed.

Earlier I saw that I had list(some_dict) in some code. Not sure why but
maybe because it's the same in Python 2 and 3.

Oscar
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