diff --git a/searches/binary_tree_traversal.py b/searches/binary_tree_traversal.py
index 47af57f7f94d..5e5a276f5a7a 100644
--- a/searches/binary_tree_traversal.py
+++ b/searches/binary_tree_traversal.py
@@ -259,6 +259,20 @@ def post_order_iter(node: TreeNode) -> None:
 
 
 def prompt(s: str = "", width=50, char="*") -> str:
+    """Return a prompt string padded to the specified width.
+
+    >>> [prompt("Python", width=width) for width in range(8, 13)]
+    [' Python ', ' Python *', '* Python *', '* Python **', '** Python **']
+
+    >>> prompt("Python", width=40, char=chr(0x1F40D))
+    '🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍 Python 🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍🐍'
+
+    >>> len(prompt("Python"))
+    50
+
+    >>> prompt("Python", -200)
+    ' Python '
+    """
     if not s:
         return "\n" + width * char
     left, extra = divmod(width - len(s) - 2, 2)