Dec -> base27

convert.js

function checkletter (s) {
	lcount = 26; // 26 letters
	var i = 0;
	var csum = s.split('').reduce(function(sm, c, idx) {
		var n = c.charCodeAt(0)-65;
		if (idx % 2 == 0) {
			return sm + n;
		} else {
			return sm + n*2;
		}
	}, 0);
	var check = lcount - (csum % lcount);
	return String.fromCharCode(check+64);
}

function base27_to_dec_with_check(b) {
	var b_string = b.toUpperCase();
	b = b_string.split('');
	var base = 26; // 26 letters
	var cl = b.pop();
	if (cl !== checkletter(b.join(''))) {
		throw new Error("Invalid checksum!");
	}
	return b.reverse().reduce(function (dec, c, idx) {
		return dec + (c.charCodeAt(0) - 65) * Math.pow(base, idx);
	}, 0);
}



function dec_to_base27_with_check(n) {
	var base = 26;
	var s = "";
	while (n != 0) {
		var r = n % base;
		s = String.fromCharCode(r+65) + s;
		n = (n / base) >> 0;
	}
	l = checkletter(s);
	return (s + l).toUpperCase();
}

convert.py

def checkletter(s):
    """Calculate checkletter with Luhn algorithm https://en.wikipedia.org/wiki/Luhn_algorithm"""
    digits = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    sm = 0
    i = 0
    for x in s:
        if i%2==0:
            sm += digits.index(x)
        else:
            sm += digits.index(x)*2
        i += 1
    check = len(digits) - (sm % len(digits))
    return digits[check-1]

def dec_to_base27_with_check(n):
    """convert positive decimal integer to base27 code + checkletter"""

    digits = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    base = len(digits)
    s = ""
    while 1:
        r = n % base
        s = digits[r] + s
        n = n / base
        if n == 0:
            break
    l = checkletter(s)
    return (s + l).upper()



def base27_to_dec_with_check(b):
    """convert base27 code with checkletter to decimal integer"""
    b = b.upper()
    digits = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    base = len(digits)
    b_without_checkletter = b[:-1].upper()
    l = checkletter(b_without_checkletter)
    if l != b[-1]:
        raise ArithmeticError("Checksum is incorrect!")
    i = 0
    dec = 0
    for c in b_without_checkletter[::-1]:
        dec += digits.index(c)*(base**i)
        i+=1
    return dec


print dec_to_base27_with_check(123987)
print base27_to_dec_with_check("HBKTU")