Question

What would be an elegant and pythonic way to implement cumsum?
Alternatively – if there’a already a built-in way to do it, that would be even better of course…

Answer 1

It’s available in Numpy:

>>> import numpy as np
>>> np.cumsum([1,2,3,4,5])
array([ 1,  3,  6, 10, 15])

Or use itertools.accumulate since Python 3.2:

>>> from itertools import accumulate
>>> list(accumulate([1,2,3,4,5]))
[ 1,  3,  6, 10, 15]

If Numpy is not an option, a generator loop would be the most elegant solution I can think of:

def cumsum(it):
    total = 0
    for x in it:
        total += x
        yield total

Ex.

>>> list(cumsum([1,2,3,4,5]))
>>> [1, 3, 6, 10, 15]