from functools import reduce

def str2float(s):

    dict = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
    f = s.index('.')
    #找到小数点在第几位，返回值为f
    g = len(s) - 1 - f
    #计算出小数点后有几位数
    s = s[:f] + s[f+1:]
    #把'.'从s中去掉
    def str_int(x):
        return dict[x]

    def add(a,b):
        return a * 10 + b
    n = reduce(add,(map(str_int,s)))
    #和str2int同理
    f_n = n / 10 ** (g)
    #n要比我们要求得值大10**g倍数
    return f_n