python二分搜索解法

import sys
for input in sys.stdin:
    input=float(input)
    sign=(input>0)#似乎不需要处理负数就能通过测试
    if not sign:
        input=-input
    test=input/2
    up=input
    down=0
    #for i in range(10):
    while True :
        if up-down<0.01:
            print(round(test,1))
            break
        if test*test*test <input:
            #print(test)
            down=test
            test=((test+up)/2)
        elif test*test*test >input:
            #print(test)
            up=test
            test=((test+down)/2)