Using generators in Python

generator-sum.py

# a generator that yields items instead of returning a list
def firstn(n):
    num = 0
    while num < n:
        yield num
        num += 1

sum_of_first_n = sum(firstn(1000000))

sum-of-numbers.py

# Build and return a list
def firstn(n):
  num, nums = 0, []
  while num < n:
    nums.append(num)
    num += 1
    return nums
   
sum_of_first_n = sum(firstn(1000000))