a = [None, 1, 2, 3, None, 4, None, None]
start = next(ele for ele in a if ele is not None)
for ind, ele in enumerate(a):
    if ele is None:
        a[ind] = start
else:
    start = ele
print(a)
[1, 1, 2, 3, 3, 4, 4, 4]
Заменить None в списке крайним левым значением non none