[Python-ideas] Sequence views

[Matt Gilson]

On Tue, Jul 19, 2016 at 7:48 AM, Serhiy Storchaka <storchaka at gmail.com>
wrote:

> On 18.07.16 01:21, Wes Turner wrote:
>
>> There are a number of generic implementations of these sequence
>> algorithms:
>>
>> * http://toolz.readthedocs.io/en/latest/api.html#itertoolz
>> * https://github.com/kachayev/fn.py#itertools-recipes
>> * http://funcy.readthedocs.io/en/stable/seqs.html
>>    * http://docs.python.org/2/reference/expressions.html#slicings
>>    * https://docs.python.org/2/library/itertools.html#itertools.islice
>>    * https://docs.python.org/3/library/itertools.html#itertools.islice
>>
>
> Aren't all these implementations works with iterables and iterators?
>
> On Jul 17, 2016 3:23 PM, "Serhiy Storchaka"
>> <storchaka at gmail.com
>> <mailto:storchaka at gmail.com>> wrote:
>>  >
>>  > Maybe it's time to add a new module for sequence-specific functions
>> (seqtools?). It should contain at least two classes or fabric functions:
>>  >
>>  > 1. A view that represents a sliced subsequence. Lazy equivalent of
>> seq[start:end:step]. This feature is implemented in third-party module
>> dataview [1].
>>
>> islice?
>>
>
> The result of itertools.islice() is not a sequence.


Additionally, itertools.islice doesn't support negative indexing and it's
O(start) to get the first element rather than the O(1) that it could be for
sequences.

>>> import itertools
>>> x = range(30)
>>> itertools.islice(x, -2, None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Indices for islice() must be None or an integer: 0 <= x <=
maxint.

With that said, I've never really worked with sequences that are big enough
for the runtime complexity of normal python slicing to actually matter.  I
have a feeling that in the typical case, wrapping more abstraction around
slicing and creating lazy "views" wouldn't lead to any practical
performance benefits.  For the cases where it *does* lead to practical
performance benefits, `numpy` starts to look a whole lot more attractive as
an option.  I wonder how many applications would actually benefit from this
but can't/shouldn't switch to `numpy` due to other constraints?



>
>  > 2. A view that represents a linear sequence as 2D array. Iterating
>> this view emits non-intersecting chunks of the sequence. For example it
>> can be used for representing the bytes object as a sequence of 1-byte
>> bytes objects (as in 2.x), a generalized alternative to iterbytes() from
>> PEP 467 [2].
>>
>> partition?
>>
>> http://toolz.readthedocs.io/en/latest/api.html#toolz.itertoolz.partition
>> _all
>>
>
> toolz.itertoolz.partition() is just a generator.
>
>
>
> _______________________________________________
> 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/
>



-- 

[image: pattern-sig.png]

Matt Gilson // SOFTWARE ENGINEER

E: matt at getpattern.com // P: 603.892.7736

We’re looking for beta testers.  Go here
<https://www.getpattern.com/meetpattern> to sign up!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20160719/5ac8b9fb/attachment-0001.html>