python implementation of mod10 check digit for invoices etc

mod10_algo.py

# 12345 -> '54321'  [the returned value is a string]
reverse = lambda x: str(x)[::-1]

# translate each element of x to a string and join eg. [18, 3, 2] -> '1832'
strjoin = lambda x: "".join(map(str, x))

# '1832' -> 1 + 8 + 3 + 2 -> 14
sumstr = lambda x: sum(int(n) for n in x)

# compute the number which must be added to `x` to get to the nearest 10
# eg 18 -> 2 (20-18=2)  or 43 -> 7 (50-43=7)
mod_ten = lambda x: int((10 - (x % 10)) % 10)

def doubler(strnum):
    '''yield double for every other digit
    '''
    for i, n in enumerate(strnum):
        n = int(n)
        yield n if i % 2 else n*2 

def check_digit(num):
    '''compute the check digit for a given number
    
    >>> check_digit(1234)
    4
    >>> check_digit(1236)
    9
    '''
    rnum = reverse(num)
    v = [j for j in doubler(rnum)]
    value = sumstr(strjoin(v))
    return mod_ten(value)

def checked_num(num):
    '''return invoice number formatted with its check-digit

    >>> checked_num(1502237)
    '1502237-9'
    '''
    return "%d-%d" % (num, check_digit(num))

def check(invoice_number):
    '''verifiy that a given invoice number (with its check digit) checks out
    
    >>> check('1502237-9')
    True
    >>> # notice the transposition from the above
    >>> check('1502273-9')
    False
    '''
    n, c = invoice_number.split('-')
    return check_digit(int(n)) == int(c)