def str2float(s):

    s1 = s[:s.find('.')]

    s2 = s[s.find('.')+1:]

    reduce(lambda x, y: x*10 + y, map(int, s1))

    reduce(lambda x, y: x/10 +y, map(int, s2[-1::-1]))

    return reduce(lambda x, y: x*10 + y, map(int, s1)) + reduce(lambda x, y: x/10 +y, map(int, s2[-1::-1]))/10

str2float('123.456')

123.456

if abs(str2float('123.456') - 123.456) < 0.00001:

    print('测试成功!')

else:

    print('测试失败!')

测试成功!