convert a string to integer without int() when the string is alphanumeric convert until find the first letter

atoit.py

def atoi(s):
    integer = 0
    sign = 1
    s0 = s[0]
    if s0 in '+-':
        s = s[1:]
        if s0 == '-':
            sign = -1
    
    for d in s:
        if '0' <= d <= '9':
            integer=integer*10 + ord(d) - ord('0')
        else:
            break
        
    return sign*integer

#Examples
#print(atoi('-123'))  -> -123
#print(atoi('+123'))  -> 123
#print(atoi('12a13')) -> 12