python - List comprehension for running total

Question

I want to get a running total from a list of numbers.

For demo purposes, I start with a sequential list of numbers using range

a = range(20)

runningTotal = []
for n in range(len(a)):
    new = runningTotal[n-1] + a[n] if n > 0 else a[n]
    runningTotal.append(new)

# This one is a syntax error
# runningTotal = [a[n] for n in range(len(a)) if n == 0 else runningTotal[n-1] + a[n]]

for i in zip(a, runningTotal):
    print "{0:>3}{1:>5}".format(*i)

yields

  0    0
  1    1
  2    3
  3    6
  4   10
  5   15
  6   21
  7   28
  8   36
  9   45
 10   55
 11   66
 12   78
 13   91
 14  105
 15  120
 16  136
 17  153
 18  171
 19  190

As you can see, I initialize an empty list [], then append() in each loop iteration. Is there a more elegant way to this, like a list comprehension?

Answer 1

A list comprehension has no good (clean, portable) way to refer to the very list it's building. One good and elegant approach might be to do the job in a generator:

def running_sum(a):
  tot = 0
  for item in a:
    tot += item
    yield tot

to get this as a list instead, of course, use list(running_sum(a)).