from itertools import accumulate

​

data = [4, 5, 12, 8, 1, 10, 21]

cumulative = list(accumulate(data, initial=100))

print(cumulative)

# [100, 104, 109, 121, 129, 130, 140, 161]

​

print([y - x for x, y in more_itertools.pairwise(cumulative)])

# [4, 5, 12, 8, 1, 10, 21]