binary search by Python3

 1 def binary_search(number, array=[]):
 2     """ binary search. """
 3     low = 0
 4     high = len(array) - 1
 5     while low <= high:
 6         mid = (low + high) // 2
 7         guess = array[mid]
 8         if guess == number:
 9             return mid
10         elif guess < number:
11             low = mid + 1
12         else: #guess > number
13             high = mid - 1
14     return None