[Python-ideas] Proposal: A Reduce-Map Comprehension and a "last" builtin

[Steven D'Aprano]

On Thu, Apr 05, 2018 at 05:31:41PM -0700, Ethan Furman wrote:
> On 04/05/2018 03:24 PM, Peter O'Connor wrote:
> 
> >Well, whether you factor out the loop-function is a separate issue.  Lets 
> >say we do:
> >
> >     smooth_signal = [average = compute_avg(average, x) for x in signal 
> >     from average=0]
> >
> >Is just as readable and maintainable as your expanded version, but saves 4 
> >lines of code.  What's not to love?
> 
> It is not readable and it is not Python (and hopefully never will be).

Be fair. Strip out the last "from average = 0" and we have little that 
isn't either in Python or is currently being proposed elsewhere. Change 
the syntax for assignment within the comprehension to one of the 
preferred syntax variants from last month's "Statement local name 
bindings" thread, and we have something that is strongly being 
considered:

    [(average := compute_avg(average, x)) for x in signal]

    [(compute_avg(average, x) as average) for x in signal]

All we need now is a way to feed in the initial value for average. And 
that could be as trival as assigning a local name for it:

    average = 0

before running the comprehension.



-- 
Steve