from functools import reduce
DIGITS = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
def str2float(s):
    n = - len(s) + s.find('.') + 1
    s = [p for p in s if p != '.']
    f = reduce(lambda x, y: 10 * x + y, map(lambda m: DIGITS[m], s)) * 10**n
    return f

print('str2float(\'123.456\') =', str2float('123.456'))
if abs(str2float('123.456') - 123.456) < 0.00001:
    print('测试成功!')
else:
    print('测试失败!')