python计算一段字符串字母的和（a，b分别为1，2）

from string import lowercase
# print lowercase
a = raw_input("Enter a string:")
a_lower = a.lower()
# print a_lower
def alpha_sum(s):
    total=0
    for i in s:
        if i in lowercase:
            total += (lowercase.index(i)+1)
        else:
            pass
    return total
print alpha_sum(a_lower)