def calc_numbers1(number):
    res = 0
    divisor = 1
    while divisor < number:
        res += (number % (divisor*10)) // (divisor)
        divisor *= 10
    return res

def calc_numbers2(number):
    res = 0
    while number:
        res += number % 10
        number //= 10
    return res

def andrej(number):
    return sum(int(i) for i in str(number))

def ed(number):
    return sum(map(int, str(number)))

def calc_numbers1(number):
    res = 0
    divisor = 1
    while divisor < number:
        res += (number % (divisor*10)) // (divisor)
        divisor *= 10
    return res

def calc_numbers2(number):
    res = 0
    while number:
        res += number % 10
        number //= 10
    return res

def calc_numbers3(number):
    res = 0
    while number:
        res = res + number % 10
        number = number // 10
    return res

In [3]: timeit andrej(1234567)
100000 loops, best of 3: 9.15 us per loop

In [4]: timeit ed(1234567)
100000 loops, best of 3: 8.54 us per loop

In [5]: timeit calc_numbers1(1234567)
100000 loops, best of 3: 3.19 us per loop

In [6]: timeit calc_numbers2(1234567)
100000 loops, best of 3: 2.02 us per loop

In [7]: timeit calc_numbers3(1234567)
100000 loops, best of 3: 2.03 us per loop

def digit_sum(n):
    s = str(n)
    return sum(map(ord, s)) - len(s)*ord('0')