Binary Search refactoring

BinarySeach.py

@@ -1,30 +1,77 @@
-def binarySearch(alist, item):
-    
-    first = 0
-    last = len(alist)-1
-    found = False
-    
-    while first<=last and not found:
-        
-        midpoint = (first + last)//2
-        if alist[midpoint] == item:
-             found = True
-             print("Found [ at position: %s ]" % (alist.index(item) + 1))
-        else:
-            
-            if item < alist[midpoint]:
-                
-                last = midpoint-1
-            else:
-                first = midpoint+1
-                
-        if found == False:
-            continue
-            # print("Not found")
-    return found
-print("Enter numbers seprated by space")
-s = input()
-numbers = list(map(int, s.split()))
-trgt =int( input('enter a single number to be found in the list '))
-binarySearch(numbers, trgt)
+"""
+This is pure python implementation of binary search algorithm
 
+For doctests run following command:
+python -m doctest -v selection_sort.py
+or
+python3 -m doctest -v selection_sort.py
+
+For manual testing run:
+python binary_search.py
+"""
+from __future__ import print_function
+
+
+def binary_search(sorted_collection, item):
+    """Pure implementation of binary sort algorithm in Python
+
+    :param sorted_collection: some sorted collection with comparable items
+    :param item: item value to search
+    :return: index of found item or None if item is not found
+
+    Examples:
+    >>> binary_search([0, 5, 7, 10, 15], 0)
+    0
+
+    >>> binary_search([0, 5, 7, 10, 15], 15)
+    4
+
+    >>> binary_search([0, 5, 7, 10, 15], 5)
+    1
+
+    >>> binary_search([0, 5, 7, 10, 15], 6)
+
+    >>> binary_search([5, 2, 1, 5], 2)
+    Traceback (most recent call last):
+    ...
+    ValueError: Collection must be sorted
+    """
+    if sorted_collection != sorted(sorted_collection):
+        raise ValueError('Collection must be sorted')
+    left = 0
+    right = len(sorted_collection) - 1
+
+    while left <= right:
+        midpoint = (left + right) // 2
+        current_item = sorted_collection[midpoint]
+        if current_item == item:
+            return midpoint
+        else:
+            if item < current_item:
+                right = midpoint - 1
+            else:
+                left = midpoint + 1
+    return None
+
+
+if __name__ == '__main__':
+    import sys
+    # For python 2.x and 3.x compatibility: 3.x has not raw_input builtin
+    # otherwise 2.x's input builtin function is too "smart"
+    if sys.version_info.major < 3:
+        input_function = raw_input
+    else:
+        input_function = input
+
+    user_input = input_function('Enter numbers separated by coma:\n')
+    collection = [int(item) for item in user_input.split(',')]
+
+    target_input = input_function(
+        'Enter a single number to be found in the list:\n'
+    )
+    target = int(target_input)
+    result = binary_search(collection, target)
+    if result is not None:
+        print('{} found at positions: {}'.format(target, result))
+    else:
+        print('Not found')