[Python-ideas] Extremely weird itertools.permutations

[Neil Girdhar]

Why not just use the standard python way to generalize this: "key" rather
than the nonstandard "filter_by".


On Sat, Oct 12, 2013 at 1:38 AM, David Mertz <mertz at gnosis.cx> wrote:

> Hi Andrew,
>
> I've sort of said as much in my last reply to Nick.  But maybe I can
> clarify further.  I can imagine *someone* wanting a filtering of
> permutations by either identify or equality.  Maybe, in fact, by other
> comparisons also for generality.
>
> This might suggest an API like the following:
>
>   equal_perms = distinct_permutations(items, r, filter_by=operator.eq)
>   ident_perms = distinct_permutations(items, r, filter_by=operator.is_)
>
> Or even perhaps, in some use-case that isn't clear to me, e.g.
>
>   start_same_perms = distinct_permutations(items, r, filter_by=lambda a,b:
> a[0]==b[0])
>
> Or perhaps more plausibly, some predicate that, e.g. tests if two returned
> tuples are the same under case normalization of the strings within them.
>
> I guess the argument then would be what the default value of 'filter_by'
> might be... but that seems less important to me if there were an option to
> pass a predicate as you liked.
>
>
>
> On Fri, Oct 11, 2013 at 7:57 PM, Andrew Barnert <abarnert at yahoo.com>wrote:
>
>> On Oct 11, 2013, at 19:48, David Mertz <mertz at gnosis.cx> wrote:
>>
>> > My feeling, however, is that if one were to trim down the results from
>> a permutations-related function, it is more interesting to me to only
>> eliminate IDENTICAL items, not to eliminate merely EQUAL ones.
>>
>> I agree with the rest of your message, but I still think you're wrong
>> here. Anyone who is surprised by distinct_permutations((3.0, 3)) treating
>> the two values the same would be equally surprised by {3.0, 3} having only
>> one member. Or by groupby((3.0, 'a'), (3, 'b')) only having one group. And
>> so on.
>>
>> In Python, sets, dict keys, groups, etc. work by ==. That was a choice
>> that could have been made differently, but Python made that choice long
>> ago, and has applied it completely consistently, and it would be very
>> strange to choose differently in this case.
>
>
>
>
> --
> Keeping medicines from the bloodstreams of the sick; food
> from the bellies of the hungry; books from the hands of the
> uneducated; technology from the underdeveloped; and putting
> advocates of freedom in prisons.  Intellectual property is
> to the 21st century what the slave trade was to the 16th.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20131012/4ee5dd57/attachment.html>