Sume Every 10 element in the list and Divided of Sum

[quest_]

A generator that will do this

def divbysum(items, step=10):
    s=1
    for i, value in enumerate(items):
        if i % step == 0:
            s = sum(items[i:i+step])
        yield value/s

source = list(range(20))
result = [x for x in divbysum(source, 5)]
print(result)

Thank you very much for the answer.
I have a question
Why do we have 5 in this line

result = [x for x in divbysum(source, 5)]