def _sum_list(list_inn):
    """Returns a new list where every entry is the sum of entries thus far in list_inn."""

    sum_this_far = 0
    sum_list = []
    for l in list_inn:
        if l is None:
            sum_list.append(None)
        else:
            sum_this_far += l
            sum_list.append(sum_this_far)

    return sum_list