add string numbers

add_string_nums.py

def add_string_nums(s1, s2):
    if s1 is None or s2 is None:
        raise AttributeError()
    val = 0
    rev_s1 = s1[::-1]
    rev_s2 = s2[::-1]
    for i in range(len(s1)):
        curr = ord(rev_s1[i])
        if curr >= 48 and curr <= 57:
            val += (curr - 48) * 10 ** i
        else:
            raise AttributeError()
    for i in range(len(s2)):
        curr = ord(rev_s2[i])
        if curr >= 48 and curr <= 57:
            val += (curr - 48) * 10 ** i
        else:
            raise AttributeError()
    return val

print(add_string_nums("1002", "4001"))
print(add_string_nums("10", "1"))
print(add_string_nums("1008", "12"))
print(add_string_nums("9999", "9999"))
print(add_string_nums("11111", "111111"))
print(add_string_nums("0", "1"))
print(add_string_nums("1000000000", "1"))
try:
    print(add_string_nums("a", "1"))
except AttributeError:
    print("That's not gonna work")