Construct String from Binary Tree

class Solution(object):
    def tree2str(self, root):
        def dfs(root):
            if not root:
                return ""
            left =  dfs(root.left) 
            right = dfs(root.right)
            if not left and not right:
                return str(root.val)
            right = "(" + right  + ")" if right else ""
            new = str(root.val) + "(" + left + ")" + right
            return  new
        return dfs(root)