Alright hear me out here:
I've often found that it would be useful for the following type of
expression to be condensed to a one-liner:
def running_average(x_seq):
averages = []
avg = 0
for t, x in enumerate(x_seq):
avg = avg*t/(t+1) + x/(t+1)
averages.append(avg)
return averages
Because really, there's only one line doing the heavy lifting here, the
rest is kind of boilerplate.
Then I learned about the beautiful and terrible "for x in [value]":
def running_average(x_seq):
return [avg for avg in [0] for t, x in enumerate(x_seq) for avg in
[avg*t/(t+1) + x/(t+1)]]
Many people find this objectionable because it looks like there are 3 for
loops, but really there's only one: loops 0 and 2 are actually assignments.
**My Proposal**
What if we just officially bless this "using for as a temporary assignment"
arrangement, and allow "for x=value" to mean "assign within the scope of
this for". It would be identical to "for x in [value]", just more
readable. The running average function would then be:
def running_average(x_seq):
return [avg for avg=0 for t, x in enumerate(x_seq) for avg = avg *
t/(t+1) + x / (t+1)]
------ P.S. 1
I am aware of Python 3.8's new "walrus" operator, which would make it:
def running_average(x_seq):
avg = 0
return [avg := avg*t/(t+1) + x / (t+1) for t, x in enumerate(x_seq)]
But it seems ugly and bug-prone to be initializing a in-comprehension
variable OUTSIDE the comprehension.
------ P.S. 2
The "for x = value" syntax can achieve things that are not nicely
achievable using the := walrus. Consider the following example (wherein we
carry forward a "hidden" variable h but do not return it):
y_seq = [y for h=0 for x in x_seq for y, h = update(x, h)]
There's not really a nice way to do this with the walrus because you can't
(as far as I understand) combine it with tuple-unpacking. You'd have to do
something awkward like:
yh = None, 0
y_seq, _ = zip(*(yh := update(x, yh[1]) for x in x_seq))
------
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