140 – Compute partial sums with accumulate

itertools.accumulate computes partial sums of the given iterable. Or, in other words, accumulate computes prefix sums.

For example, if you give it a list of bank movements, accumulate computes the intermediate bank balances:

from itertools import accumulate

movements = [100, -200, 300, -450]
initial_balance = 1000

for balance in accumulate(movements, initial=initial_balance):
    print(balance)
# 1000
# 1100 (1000 + 100)
# 900  (1100 - 200)
# 1200 (900 + 300)
# 750  (1200 - 450)

This shows there is a relationship between accumulate and the built-in sum if you consider slices of the list movements with the initial balance at the front:

    [1000, 100, -200, 300, -450]
sum(     )                         # 1000
sum(          )                    # 1100
sum(                )              # 900
sum(                     )         # 1200
sum(                            )  # 750