return -1 if target is not in array (python)

utilities/python/binary_search.py

@@ -9,14 +9,17 @@ def binary_search(arr, target):
             left = mid + 1
         else:
             right = mid - 1
-    return left
+    if arr[left] == target:
+        return left
+    else:
+        return -1
 
 print(binary_search([1, 2, 3, 10], 1) == 0)
 print(binary_search([1, 2, 3, 10], 2) == 1)
 print(binary_search([1, 2, 3, 10], 3) == 2)
 print(binary_search([1, 2, 3, 10], 10) == 3)
-print(binary_search([1, 2, 3, 10], 9) == 3)
-print(binary_search([1, 2, 3, 10], 4) == 3)
-print(binary_search([1, 2, 3, 10], 0) == 0)
-print(binary_search([1, 2, 3, 10], 11) == 3)
-print(binary_search([5, 7, 8, 10], 3) == 0)
+print(binary_search([1, 2, 3, 10], 9) == -1)
+print(binary_search([1, 2, 3, 10], 4) == -1)
+print(binary_search([1, 2, 3, 10], 0) == -1)
+print(binary_search([1, 2, 3, 10], 11) == -1)
+print(binary_search([5, 7, 8, 10], 3) == -1)