diff --git a/data_structures/binary_tree/fenwick_tree.py b/data_structures/binary_tree/fenwick_tree.py
index 54f0f07ac68d..1ae3b14d60b1 100644
--- a/data_structures/binary_tree/fenwick_tree.py
+++ b/data_structures/binary_tree/fenwick_tree.py
@@ -1,28 +1,42 @@
 class FenwickTree:
-    def __init__(self, SIZE):  # create fenwick tree with size SIZE
+    """
+    >>> f = FenwickTree(100)
+    >>> f.update(1, 20)
+    >>> f.update(4, 4)
+    >>> f.query(1)
+    20
+    >>> f.query(3)
+    20
+    >>> f.query(4)
+    24
+    >>> f.update(2, -5)
+    >>> f.query(1)
+    20
+    >>> f.query(3)
+    15
+    """
+
+    def __init__(self, SIZE: int):  # create fenwick tree with size SIZE
         self.Size = SIZE
-        self.ft = [0 for i in range(0, SIZE)]
+        self.ft = [0] * SIZE
+
+    def __lowbit(self, i: int) -> int:
+        return i & (-i)
 
-    def update(self, i, val):  # update data (adding) in index i in O(lg N)
+    def update(self, i: int, val):  # update data (adding) in index i in O(lg N)
         while i < self.Size:
             self.ft[i] += val
-            i += i & (-i)
+            i += self.__lowbit(i)
 
-    def query(self, i):  # query cumulative data from index 0 to i in O(lg N)
+    def query(self, i: int):  # query cumulative data from index 0 to i in O(lg N)
         ret = 0
         while i > 0:
             ret += self.ft[i]
-            i -= i & (-i)
+            i -= self.__lowbit(i)
         return ret
 
 
 if __name__ == "__main__":
-    f = FenwickTree(100)
-    f.update(1, 20)
-    f.update(4, 4)
-    print(f.query(1))
-    print(f.query(3))
-    print(f.query(4))
-    f.update(2, -5)
-    print(f.query(1))
-    print(f.query(3))
+    from doctest import testmod
+
+    testmod()