[Python-ideas] Vectorization [was Re: Add list.join() please]

[Steven D'Aprano]

On Sat, Feb 02, 2019 at 03:31:29PM -0500, David Mertz wrote:

> I still haven't seen any examples that aren't already spelled 'map(fun, it)'

You might be right. But then there's nothing that map() can do that 
couldn't be written as a comprehension, and nothing that you can't do 
with a comprehension that can't be written as a for-loop.

And nothing that can't be written as a for-loop that couldn't be written 
as a while-loop. The only loop construct we really need is a while loop. 
And even that is redundant if we had GOTO.

Its not about the functionality, but expressibility and readability.

This hypothetical vectorization syntax might have a performance 
advantage as well. My understanding is that Julia is able to efficiently 
vectorize code, bringing it to within 10% of the speed of unrolled C 
loops. It may be that CPython cannot do anything that fast, but there 
may be some opportunities for optimization that we cannot apply to 
for-loops or comprehensions due to the way they are defined.

But primarily it is about the readability of the code:

    result = process.(vector .+ sequence) .* items

versus:

    # Ouch!
    result = map(operator.mul, 
                 zip(map(process, 
                         map(operator.add,
                             zip(vector, sequence)), 
                 items))


Here's the comprehension version:

    result = [a*b for a, b in zip(
                 [process(c) for c in 
                     [d+e for d, e in zip(vector, sequence)]], 
                 items)]

We can improve that comprehension a tiny bit by splitting it into 
multiple steps:

     temp1 = [d+e for d, e in zip(vector, sequence)]
     temp2 = [process(c) for x in temp1]
     result = [a*b for a, b in zip(temp2, items)]

but none of these are as elegant or readable as the vectorized syntax

     result = process.(vector .+ sequence) .* items



-- 
Steve