[Python-ideas] Add nullifier argument to functools.reduce?

[David Mertz]

Completely off topic, but the symbol substitutions in
https://github.com/ehamberg/vim-cute-python/tree/moresymbols used '∅' for
both 'set()' and 'None' which seemed confusing to me.  Using aleph ('ℵ')
both somewhat resembles a Roman 'N(one)' and also makes a certain kind of
sense if you think of aleph0 as the first "inaccessible cardinal" in set
theory.

It's too bad I didn't have any loops or 'in' tests in my three liner, since
I love the look of 'i ∈ collection' on screen.


On Sat, Aug 23, 2014 at 11:43 AM, David Mertz <mertz at gnosis.cx> wrote:

> It's true, Steven, that we'd have to use itertools.accumulate() rather
> than functools.reduce() to do the heavy lifting here (as Bob suggested).
>  But using that, here are three lines:
>
>   from itertools import *
>   from operator import add
>
>   def reduce_with_attractor(func, it, start=ℵ, end_if=ℵ):
>       it = iter(it)
>       start = start if start≢ℵ else it.__next__()
>       return list(takewhile(λ x: x≢end_if,
>                             accumulate(chain([start],it), func)))[-1]
>
> Oh, that's cute... my copy-paste was using
> https://github.com/ehamberg/vim-cute-python (somewhat customized further
> by me).  In regular ASCII:
>
>   def reduce_with_attractor(func, it, start=None, end_if=None):
>       it = iter(it)
>       start = start if start!=None else it.__next__()
>       return list(takewhile(lambda x: x!=end_if,
>                             accumulate(chain([start],it), func)))[-1]
>
> This gives you the accumulation up-to-but-not-including the attractor.  I
> guess the OP wanted to return the attractor itself (although that seems
> slightly less useful to me).  I'd have to play around to get that version
> in three lines... maybe it would take 4 or 5 lines to do it.
>
>
> On Sat, Aug 23, 2014 at 10:43 AM, Steven D'Aprano <steve at pearwood.info>
> wrote:
>
>> On Sat, Aug 23, 2014 at 10:25:06AM -0700, David Mertz wrote:
>> > This "nullifier" is mathematically called an "absorbing element", but
>> > saying an "attractor" might be a little more general.  I.e. think of a
>> > local optimization problem, where multiple local min/max points might
>> > occur.  If you reach one, further iteration won't budge from that point,
>> > even if it's not the "global absorbing element."
>>
>> Yes. Note that arbitrary systems may have more than one attractors,
>> including cycles (a -> b -> c -> a) and even "strange attractors" of
>> infinite complexity. It's probably too much to expect reduce to deal
>> with cycles, but a really general solution should at least deal with
>> multiple attractors.
>>
>> > Given that one can easily write one's own three line wrapper
>> > 'reduce_with_attractor()' for this special semantics
>>
>> I don't think you can. Although it's 3:30am here and it's past my bed
>> time, so perhaps I'm wrong. The problem is that the wrapper cannot see
>> the reduced value until reduce() returns, and we want to short-circuit
>> the call once the reduced value is the attractor.
>>
>> Still, easy or not, I think the semantics are too specialised to
>> justify in the standard library, especially given that Guido doesn't
>> like reduce and it almost got removed. A better solution, I think, would
>> be to stick this reduce_with_attractor() in some third-party functional
>> tool library.
>>
>>
>> --
>> Steven
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>
>
>
> --
> 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.
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