def bin_search(list,item):
    low=0
    high=len(list)-1
    while low<=high:            #4
        mid = round(((low + high) / 2)+0.1,0) #1
        #mid=(low + high) / 2
        guess=list[int(mid)]
        if guess==item:
         return int(mid)
        if guess>item:
            high=mid-1    #2
        else:
 

            low=mid+1    #3
    return None

my_list=[1,3,5,7,9]
print(bin_search(my_list,7))

註意二分法的思想，實現中四處標記的原理，以及第四處加等號和不加等號的影響

二分法python實現