您发布的两行都很好，但您可以纯粹用整数来完成，这样效率最高：

def sum_digits(n):
    s = 0
    while n:
        s += n % 10
        n //= 10
    return s

或者divmod：

def sum_digits2(n):
    s = 0
    while n:
        n, remainder = divmod(n, 10)
        s += remainder
    return s

稍微快一点的是使用单个赋值语句：

def sum_digits3(n):
   r = 0
   while n:
       r, n = r + n % 10, n // 10
   return r

> %timeit sum_digits(n)
1000000 loops, best of 3: 574 ns per loop

> %timeit sum_digits2(n)
1000000 loops, best of 3: 716 ns per loop

> %timeit sum_digits3(n)
1000000 loops, best of 3: 479 ns per loop

> %timeit sum(map(int, str(n)))
1000000 loops, best of 3: 1.42 us per loop

> %timeit sum([int(digit) for digit in str(n)])
100000 loops, best of 3: 1.52 us per loop

> %timeit sum(int(digit) for digit in str(n))
100000 loops, best of 3: 2.04 us per loop