convert a string to a number

atof.py

def atof(s, base):
    def ctoi(ch):
        ch = ch.lower()
        if ch <= '9' and ch >= '0':
            return ord(ch) - ord('0')
        elif ch >= 'a' and ch <= 'z':
            return ord(ch) - ord('a') + 10
        else: 
            raise Exception('argument error: char %s is not a valid numeric symbol' % ch)
    
    if not isinstance(s, str):
        raise Exception('argument error: should be a string')
    elif len(s) == 0:
        raise Exception('argument error: empty string')
    elif len(s) == 1:
        # sanity check for: '-', '.'
        ctoi(s[0])
        
    sign = 1        
    a, b = base, 1
    res = 0
    is_frac = False
    i = 0
    
    for ch in s:
        if is_frac:
            b = float(b * base)
        if (ch <= '9' and ch >= '0') or (ch >= 'a' and ch <= 'z'):
            if ctoi(ch) >= base:
                raise Exception('string contains invalid char: %s for base: %d' % (ch, base))
            res = res * a + ctoi(ch) / b
        elif ch == '.' and is_frac is False:
            a = 1
            is_frac = True
        elif (ch == '+' or ch == '-') and i == 0:
            if ch == '-':
                sign = -1
        else:
            raise Exception('string contains invalid char: %s' % ch)
        i += 1
            
    return res * sign
            
    
# atof(123)
print atof('-2.2', 10)