This is my first time posting! But sinceSince you asked for a Pythonic solution and I see none so far, I propose:
new_L = [sum(L[:i+1]) for i in range(len(L))] It's certainly less efficient than an accumulator -- it's O((n^2)/2)\$O(\frac{n^2}{2})\$ vs O(n)\$O(n)\$ -- but it uses a list comprehension as you suggested.