is it possible to for-loop through a list with integers and use the += operator to add the integers to a variable with 0? eg:

length = [3, 2]
grades = [1, 1, 1, 5, 4]
avg_grade = []

c = 0
d= 0
r = 0
x = 0
for t in length:
    b = 0
    for s in range(d, d + t):
        b += grades[s]
        d += 1
        r += 1
        if r == length[x]:
           c = b / t
           avg_grade.append(c)
           x += 1
           r = 0

I always receive:
TypeError: unsupported operand type(s) for +=: ‘int’ and ‘types.GenericAlias’

Is there another way to calculate the average grades? Maybe with sum()?

Thanks!