from functools import reduce

def str2float(s):
    if not isinstance(s,str):
        print('错误,请输入str')
        return None
    inx = s.index('.')
    s = s.replace('.','')
    length = len(s)
    print(inx, length)
    il = length - inx
    def fn(x, y):
        return x * 10 + y

    def char2num(num):
        s = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}[num]
        return s
    return reduce(fn, map(char2num, s)) / 10**il

话说为啥不直接float('123.456')