[Python-ideas] Delayed Execution via Keyword

[David Mertz]

On Thu, Feb 16, 2017 at 11:15 PM, David Mertz <mertz at gnosis.cx> wrote:
>
> This also means that a 'delayed' object needs to be idempotent.  So
>
> x = delayed 2+2
>
> y = delayed x
>
> z = delayed delayed delayed y
>
>
> Wrapping more delays around an existing delayed object should probably
> just keep the same object rather than "doubly delaying" it.  If there is
> some reason to create separate delayed objects that isn't occurring to me,
> evaluating 'z' would still go through the multiple evaluation levels until
> it got to a non-delayed value.
>

This is sort of like how iterators "return self" and 'it = iter(it)'.

In the case of Dask, wrapping more delayed objects creates layers of these
lazy objects.  But I think it has to because it's not part of the syntax.
Actually, I guess Dask could do graph reduction without actual computation
if it wanted to.  But this is the current behavior:

>>> def unchanged(x):
...     return x
>>> a = delayed(unchanged)(42)
>>> a
Delayed('unchanged-1780fed6-f835-4c31-a86d-50015ae1449a')
>>> b = delayed(unchanged)(a)
>>> c = delayed(unchanged)(b)
>>> c
Delayed('unchanged-adc5e307-6e33-45bf-ad73-150b906e921d')
>>> c.dask
{'unchanged-1780fed6-f835-4c31-a86d-50015ae1449a': (<function
__main__.unchanged>,
  42),
 'unchanged-adc5e307-6e33-45bf-ad73-150b906e921d': (<function
__main__.unchanged>,
  'unchanged-c3282bc4-bdaa-4148-8509-9155cac83ef0'),
 'unchanged-c3282bc4-bdaa-4148-8509-9155cac83ef0': (<function
__main__.unchanged>,
  'unchanged-1780fed6-f835-4c31-a86d-50015ae1449a')}

>>> c.compute()
42


Actually Dask *cannot* know that "unchanged()" is the function that makes
no transformation on its one parameter.  From what it can see, it's just a
function that does *something*.  And I guess similarly in the proposed
syntax, anything other than a plain name after the 'delayed' would still
need to create a new delayed object.  So it's all an edge case that doesn't
make much difference.

-- 
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/20170216/90755f72/attachment.html>