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

[Rhodri James]

On 05/04/18 17:52, Peter O'Connor wrote:
> Dear all,
> 
> In Python, I often find myself building lists where each element depends on
> the last.  This generally means making a for-loop, create an initial list,
> and appending to it in the loop, or creating a generator-function.  Both of
> these feel more verbose than necessary.
> 
> I was thinking it would be nice to be able to encapsulate this common type
> of operation into a more compact comprehension.
> 
> I propose a new "Reduce-Map" comprehension that allows us to write:
> 
> signal = [math.sin(i*0.01) + random.normalvariate(0, 0.1) for i in range(1000)]
> smooth_signal = [average = (1-decay)*average + decay*x for x in signal
> from average=0.]

Ew.  This looks magic (and indeed is magic) and uses single equals 
inside the expression (inviting "=" vs "==" gumbies).  I think you are 
trying to do too much in one go, and something like this is complex 
enough that it shouldn't be in a comprehension in the first place.

> Instead of:
> 
> def exponential_moving_average(signal: Iterable[float], decay: float,
> initial_value: float=0.):
>      average = initial_value
>      for xt in signal:
>          average = (1-decay)*average + decay*xt
>          yield average
> 
> signal = [math.sin(i*0.01) + random.normalvariate(0, 0.1) for i in range(1000)]
> smooth_signal = list(exponential_moving_average(signal, decay=0.05))

Aside from unnecessarily being a generator, this reads better to me!

-- 
Rhodri James *-* Kynesim Ltd