Traceback (most recent call last):

File "python", line 14, in

File "python", line 2, in digit_sum

NameError: global name 'n' is not defined

"Write a function called digit_sum that takes a positive integer n as input and returns the sum of all that number's digits."

I think I did some mistakes and some unefficent writing, I really appriciate the help!

```
def digit_sum(num):
str(num) == n
len(n) == l
Number = 0
while l > 0:
for digits in range(l):
Number += int(num[l])
l -=1
return Number
print digit_sum(666)
```