[Python-ideas] Adding collections.abc.Ordered

[Neil Girdhar]

On Saturday, November 7, 2015 at 9:42:19 AM UTC-5, Stephen J. Turnbull 
wrote:
>
> Andrew Barnert writes: 
>  > On Nov 6, 2015, at 21:00, Stephen J. Turnbull <ste... at xemacs.org 
> <javascript:>> wrote: 
>
>  > > This ABC would be a tar pit of recriminations between class authors 
>  > > and class clients.   
>
>  > I think you've missed the point of his proposal. It doesn't matter 
>  > _what_ the order is 
>
> That's precisely what I see as problematic, unless the author of the 
> subclass is the same as the author of the code using the subclass. 
>
>  > But, whether the proposal is useful or not, it is sufficiently 
>  > specified. 
>
> I wrote inaccurately, I guess.  My contention is that it's more 
> dangerous than useful as currently specified but that could be 
> reversed with additional restrictions. 
>

It's not "dangerous"!!  Talk about hyperbole.  It's just a question of 
utility.  Personally I would like to see a much richer collections.abc for 
tests like this.
 

>
>  > How would you define the comparison function for OrderedDict in a 
>  > way that makes any semantic sense?  Or, for that matter, list? 
>
> Again, bad nomenclature, sorry.[1]  I should have used "index".  For 
> this application, I have in mind 
>
> class Ordered: 
>
>     def index(self, key): 
>         return list(z for z in self).index(key) 
>
>     def compare(self, key1, key2): 
>         return self.index(key1) < self.index(key2) 
>
> as the default implementation (except that for Amir's application it's 
> .index() that's interesting, not .compare() -- .compare() could be 
> omitted I guess, fn. [1] applies).  The problem with list would be 
> that it can have multiple entries of the same value that are not 
> adjacent, of course, but then it wouldn't work for Amir's application 
> anyway.  This is part of what I have in mind when I say 
> "underspecified". 
>
> I don't think thought about how this collections.abc.Ordered should 
> interact with objects that could be considered to represent multisets. 
> Do you? 
>

Did you read the original post?  All you need is a consistent order.  It 
doesn't matter if the same element turns up twice in order to be Ordered. 
 He can check uniqueness after the fact.
 

>
>  > Surely OrderedDict and list are perfectly reasonable types for 
>  > __slots__, despite the fact that their comparison function doesn't 
>  > exist. 
>
> If given the above renaming you still say it doesn't exist, I don't 
> understand what you're trying to say. 
>
>
>
> Footnotes: 
> [1]  As an excuse for the poor nomenclature, in my field I have to 
> worry about (pre-) ordered structures for which an index function does 
> not exist, thus "comparison function". 
>
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