Helper method to create a printable visual string representation of a binary search tree

binarytree.py

class BinaryTreeNode:
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None


class BinarySearchTree:
    def __init__(self):
        self.root = None
        # ...

    def __str__(self, node=None):
        """Return a visual string representation of this binary search tree.
        Performs depth-first traversal starting at the given node or root."""
        if node is None:
            if self.is_empty():  # Special case
                return 'root -> None'
            else:  # Start recursion at root node
                return '\n'.join(self.__str__(self.root))
        strings = []  # Accumulate separate strings since we need to pad them
        if node.right is not None:  # Descend right subtree, if it exists
            for right_string in self.__str__(node.right):
                # Left-pad strings and replace ambiguous root with right link
                strings.append(5 * ' ' + right_string.replace('->', '/-', 1))
        strings.append('-> ({})'.format(repr(node.data)))  # This node's string
        if node.left is not None:  # Descend left subtree, if it exists
            for left_string in self.__str__(node.left):
                # Left-pad strings and replace ambiguous root with left link
                strings.append(5 * ' ' + left_string.replace('->', '\\-', 1))
        return strings