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) # => 1
print binary_search(my_list, -1) # => None