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From: "Prasad, Ramit" <ramit.prasad@jpmorgan.com>
To: "python-list@python.org" <python-list@python.org>
Subject: RE: Python 3: dict & dict.keys()
Thread-Topic: Python 3: dict & dict.keys()
Thread-Index: AQHOiJKOfYycRz3X90uM6iWVTW41zZl0ahEAgAAYd4D//8Wi4IAAeYmAgADIZXA=
Date: Thu, 25 Jul 2013 16:11:47 +0000
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Terry Reedy wrote:
> 
> On 7/24/2013 4:34 PM, Prasad, Ramit wrote:
> 
> > I am still not clear on the advantage of views vs. iterators.
> 
> A1: Views are iterables that can be iterated more than once. Therefore,
> they can be passed to a function that re-iterates its inputs, or to
> multiple functions. They support 'x in view' as efficiently as possible.
> Think about how you would write the non-view equivalent of '(0,None) in
> somedict.views())'. When set-like, views support some set operations.
> For .keys, which are always set-like, these operations are easy to
> implement as dicts are based on a hashed array of keys.

Hmm, that is a change that makes some sense to me. Does the view
get updated when dictionary changes or is a new view needed? I 
assume the latter.  

> 
> Q2: What is the advantage of views vs. lists?
> 
> A2: They do not take up space that is not needed. They can be converted
> to lists, to get all the features of lists, but not vice versa.
> 
> > What makes d.viewkeys() better than d.iterkeys()? Why did they decide
> > not to rename d.iterkeys() to d.keys() and instead use d.viewkeys()?
> 
> This is historically the wrong way to phrase the question. The 2.7
> .viewxyz methods were *not* used to make the 3.x .xyz methods. It was
> the other way around. 3.0 came out with view methods replacing both list
> and iter methods just after 2.6, after a couple of years of design, and
> a year and a half before 2.7. The view methods were backported from 3.1
> to 2.7, with 'view' added to the name to avoid name conflicts, to make
> it easier to write code that would either run on both 2.7 and 3.x or be
> converted with 2to3.
> 
> A better question is: 'When 3.0 was designed, why were views invented
> for the .xyz methods rather than just renaming the .iterxyz methods. The
> advantages given above are the answer. View methods replace both list
> and iterator methods and are more flexible than either and directly or
> indirectly have all the advantages of both.
> 
> My question is why some people are fussing so much because Python
> developers gave them one thing that is better than either of the two
> things it replaces?

I personally am not "fussing" as existing functionality was preserved
(and improved). I just was not clear on the difference. Thanks for
all the detail and context.

> 
> The mis-phrased question above illustrates why people new to Python
> should use the latest 3.x and ignore 2.x unless they must use 2.x
> libraries. 2.7 has all the old stuff, for back compatibility, and as
> much of the new stuff in 3.1 as seemed sensible, for forward
> compatibility. Thus it has lots of confusing duplication, and in this
> case, triplication

I work with 2.6 so no choice there... :)

> 
> --
> Terry Jan Reedy
> 
> --
> http://mail.python.org/mailman/listinfo/python-list

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