[Python-ideas] Possible new itertool: comm()

[David Mertz]

I don't know if this clarifies anything, but I tried out such a mis-sorted
example using the actual 'comm' tool


511-tmp % comm comm1 comm2

A
B
C
E
F
J
A
K
M
N
T
U
V

512-tmp % cat comm[12]
A
B
C
E
F
J
A
K
T
U
V


A
B
M
N
T
U
V



On Wed, Jan 7, 2015 at 10:20 AM, David Mertz <mertz at gnosis.cx> wrote:

> Adding a itertools.comm() seems unnecessary, although adding it to the
> widely used more-itertools feels more reasonable.
>
> However, the problem with the functionality is that it assumes that the
> iterables given to it return pre-sorted values.  Which is fine it that
> assumption is satisfied, but one really can't know in advance of exhausting
> the iterators whether that will prove true.  The desired behavior in the
> absence of this pre-condition being satisfied is unclear to me.
>
> I could plausibly see the tool raising some exception when the
> pre-condition was violated.  Of course, in that case, we might well have
> already returned multiple elements on next(comm(it1, it2)) calls before the
> exception is encountered, and the initial values might be deceptive or
> retroactively invalidated.
>
> I could plausibly see it ignoring the non-sorted elements in some way.
> Maybe just pretend they are sorted rather than checking, returning somewhat
> ill-defined values, I'd think.  Or maybe just discarding the missorted
> values? Or flagging them with some other indicator indicating the
> missorting? I dunno.
>
> In any case, this is not something that can be applied to iterators
> generically, but only to iterators about which one makes lots of
> assumptions in advance, and hence seems ill-suited for the stdlib itertools.
>
>
> On Wed, Jan 7, 2015 at 8:09 AM, Paul Moore <p.f.moore at gmail.com> wrote:
>
>> On 7 January 2015 at 15:38, Steven D'Aprano <steve at pearwood.info> wrote:
>> > This example of using comm in Unix might help:
>> >
>> > http://www.theunixschool.com/2011/03/comm-beautiful-comparison.html
>>
>> The example on that page is the exact reason I wrote my own
>> implementation (the Windows version of comm that I have locally got
>> confused by non-ASCII content in some of the filenames I had).
>>
>> Paul
>> _______________________________________________
>> Python-ideas mailing list
>> Python-ideas at python.org
>> https://mail.python.org/mailman/listinfo/python-ideas
>> Code of Conduct: http://python.org/psf/codeofconduct/
>>
>
>
>
> --
> 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.
>



-- 
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/20150107/93c05075/attachment.html>