Python character function, where is the "Z"?

Question

I'm trying to convert numbers to excel column letters.

Code:

def num2col(num):
    letters = ''
    while num:
        mod = num % 26
        num = num // 26
        letters += chr(mod + 64)
    return ''.join(reversed(letters))

print num2col(25), num2col(26), num2col(27)    
print num2col(51), num2col(52), num2col(53)
print num2col(77), num2col(78), num2col(79)

Desired output:

Y Z AA
AY AZ BA
BY BZ CA

Actual output:

Y A@ AA
AY B@ BA
BY C@ CA

Answer 1

def num2col(num):
    letters = ''
    while num:
        num, mod = divmod(num-1, 26)
        letters += chr(mod + 65)      # 65 == ord('A')
    return ''.join(reversed(letters))

print num2col(25), num2col(26), num2col(27)    
print num2col(51), num2col(52), num2col(53)
print num2col(77), num2col(78), num2col(79)

output:

Y Z AA
AY AZ BA
BY BZ CA