[Python-ideas] Reduce/fold and scan with generator expressions and comprehensions

[Danilo J. S. Bellini]

>
> Of course. But if you want last(), why not just spell the utility function
> as I did? [...]
>
I'm not against a general "last", I just said the main idea of this thread
is the access to the previous iteration output in a list/set/dict
comprehension or generator expression.

> Actually, your code is similar to the reference implementation I wrote for
PyScanPrev, the main difference is that my "last" raises a StopIteration on
an empty input instead of an UnboundLocalError:
https://github.com/danilobellini/pyscanprev/blob/v0.1.0/pyscanprev.py#L148
When the input is a sequence, it should be optimized to get the item at the
index -1.

That works fine for any iteratable (including a list, array, etc), whether
> or not it's a reduction/accumulation.
>
Lists and arrays don't need to be traversed.

Consuming the iterator is *necessary* to get the last item. There's no way
> around that.
>
Not if there's enough information to create the last value. Perhaps on the
it = iter(range(9999999)) one can get 2 values (call next(it) twice) and
use its __length_hint__ to create the last value. But I think only
sequences should have such an optimization, not iterators.

-- 
Danilo J. S. Bellini
---------------
"*It is not our business to set up prohibitions, but to arrive at
conventions.*" (R. Carnap)
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