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.]
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))
I've created a complete proposal at: https://github.com/petered/peps/blob/master/pep-9999.rst , (and a pull-request) and I'd be interested to hear what people think of this idea.

Combined with the new "last" builtin discussed in the proposal, this would allow u to replace "reduce" with a more Pythonic comprehension-style syntax.  

- Peter