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

[Peter O'Connor]

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 <https://github.com/python/peps/pull/609>) 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
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