Python 3: dict & dict.keys()

[Prasad, Ramit]

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

This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email.