def binary_search(list,item):
low=0
high=len(list)-1
while low<=high:
mid=(low + high)/2
guess=list[mid]
if guess==item:
return mid
if guess>item:
high=mid-1
else:
low=mid+1
return None
my_list=[1,3,5,7,9]
print binary_search(my_list,3)
print binary_search(my_list,-1)