[issue33040] Make itertools.islice supports negative values for start and stop arguments for sized iterable object

[Yaroslav Nikitenko]

Yaroslav Nikitenko  added the comment:

Sorry for a typo. The paragraph before the last should read

Negative *step* fundamentally changes the algorithm:... flow[-1:None:-1].

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[issue33040] Make itertools.islice supports negative values for start and stop arguments for sized iterable object

[Yaroslav Nikitenko]

Yaroslav Nikitenko  added the comment:

I hope it's fine to add to closed topics here.

I agree with the decision that islice should not handle a special case of sized 
containers vs iterables.

However, I think that the support of negative indices in islice would be nice. 
A simple use case would be to get the last element of an iterable.
I had to write my own code for that (you may read and/or use it if needed), 
because I needed this algorithm in my analysis. Here is the link to my function 
ISlice._run_negative_islice in my architectural framework for data analysis: 
https://github.com/ynikitenko/lena/blob/master/lena/flow/iterators.py#L150 or 
on https://lena.readthedocs.io/en/latest/flow.html#lena.flow.iterators.ISlice 
(I'm afraid the correct link might change later).

I also agree that to support negative indices would require more work. Indeed, 
it seems that for each combination of positive/negative start/stop it required 
a new algorithm! I didn't implement negative steps.

However, I think that because it's not easy to implement, it would be even 
better to include that in the standard library (because it would save other 
people from writing that).

If we care about code reuse: I think that negative indices make the algorithm 
more useful, while non-trivial steps are redundant, because they can be 
implemented by a composition of a slice with step=1 with a simple slice with 
start, stop=None, None with step!=1.

Negative slice fundamentally changes the algorithm: if one wants to have the 
flow inverted, fflow[0, None, -1], one would have to store it all in memory! 
Anyway it's easy to make this in a separate function. So I think it's more 
functional to implement negative indices and discard negative steps (or 
non-trivial steps at all).

About drop_first, drop_last suggested above: I also think that they are 
redundant and would be better incorporated into islice.

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nosy: +ynikitenko

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[issue33040] Make itertools.islice supports negative values for start and stop arguments for sized iterable object

[TitanSnow]

TitanSnow  added the comment:

Now I have thought about it and realized that
it's not suitable for islice.
But there's still a common use case to
drop some elements from the beginning or
ending of a iterable, which is also a main
reason why I wanted islice to support
negative values of start and stop.
So how about adding two functions
``drop_first`` and ``drop_last``
to do such things. They can be implemented
like this, in which way they support
arbitrary iterators::

def drop_first(iterable, n=1):
for _ in range(n):
next(iterable)
for e in iterable:
yield e

def drop_last(iterable, n=1):
dq = deque()
for _ in range(n):
dq.append(next(iterable))
for e in iterable:
dq.append(e)
yield dq.popleft()

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[issue33040] Make itertools.islice supports negative values for start and stop arguments for sized iterable object

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I concur with Serhiy.  This doesn't make sense for itertools.

Marking this as closed. Thank you for the suggestion.

--
resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue33040] Make itertools.islice supports negative values for start and stop arguments for sized iterable object

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I do not think this is suitable for the itertools module. It is designed to 
work with iterators, i.e. with objects that support the iterator protocol. What 
if the iterable change the size during iteration? Should the islice iterator 
produce less or larger number of items than the initial estimation? Or detect 
this change and raise an error? Should it revive the iteration if new items 
were added after consuming the last item? These design questions should have 
thoughtful answers. And handling all corner cases will complicate the code. I 
afraid that after adding support of negative start and stop we will get a 
request for supporting negative step.

It is possible to implement support of negative indices for arbitrary iterators 
using tee() or deque, but this will complicate the islice() code even more for 
a small niche case.

This may be a use case for the new module that works specially with sequences. 
It will be added once we will have enough use cases.

--
nosy: +rhettinger, serhiy.storchaka

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[issue33040] Make itertools.islice supports negative values for start and stop arguments for sized iterable object

[TitanSnow]

New submission from TitanSnow :

``islice()`` does not support negative values for start or stop,
which does not matter for plain iterators.
However, for some cases, we have a sized iterable object
which is not subscriptable, using ``islice()`` makes
code ugly::

d = OrderedDict()
for i in range(10): d[i] = i
dv = d.keys()
# dv is a KeysView which is a sized iterable

# now I wanna get a slice of dv which does not contain the last element
islice(dv, len(dv) - 1)

As it shows, I have to use ``len(dv)`` to get its length.

For sized iterable objects, ``islice()`` could
support negative values for start or stop.
In this way, the above code can be written like this::

islice(dv, -1)

For iterable objects which is not sized,
it could still be not supported::

islice(iter(range(10)), -1)

raises a ValueError as its original behavior.

--
components: Library (Lib)
files: islice.patch
keywords: patch
messages: 313517
nosy: tttnns
priority: normal
severity: normal
status: open
title: Make itertools.islice supports negative values for start and stop 
arguments for sized iterable object
type: enhancement
Added file: https://bugs.python.org/file47475/islice.patch

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