diff --git a/data_structures/persistent_segment_tree.py b/data_structures/persistent_segment_tree.py
index e3a842791..f7d0bdbbc 100644
--- a/data_structures/persistent_segment_tree.py
+++ b/data_structures/persistent_segment_tree.py
@@ -1,11 +1,13 @@
 from typing import List, Optional
 
+
 class Node:
     def __init__(self, value: int = 0) -> None:
         self.value: int = value
         self.left: Optional[Node] = None
         self.right: Optional[Node] = None
 
+
 class PersistentSegmentTree:
     def __init__(self, arr: List[int]) -> None:
         self.n: int = len(arr)
@@ -120,12 +122,15 @@ class PersistentSegmentTree:
         if left <= start and right >= end:
             return node.value
         mid = (start + end) // 2
-        return (self._query(node.left, start, mid, left, right) +
-                self._query(node.right, mid + 1, end, left, right))
+        return self._query(node.left, start, mid, left, right) + self._query(
+            node.right, mid + 1, end, left, right
+        )
+
 
 # Running the doctests
 if __name__ == "__main__":
     import doctest
+
     print("Running doctests...")
     result = doctest.testmod()
     print(f"Ran {result.attempted} tests, {result.failed} failed.")