How to print a binary tree intuitively

Print a binary tree in a hierarchical structure, like the one below

 

class Node:
    def __init__(self, val, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

    def __str__(self):
        return str(self.val)

def prt_bin_tree(root, str_levels, level=0, output_num=0):
    if root is None:
        return 0

    def output(str_levels, node, pos, level):
        s = str_levels[level]
        def expand_str(s, n):
            l = len(s)
            while l <= n:
                s+=' '
                l+=1
            return s
        s = expand_str(s, pos-1)
        s+=str(node.val)
        str_levels[level] = s

    left_output_num = prt_bin_tree(root.left, str_levels, level+1, output_num)
    output(str_levels, root, output_num + left_output_num, level)
    right_output_num = prt_bin_tree(root.right, str_levels, level+1, output_num + left_output_num +1)

    return left_output_num + right_output_num + 1


if __name__ == "__main__":

    from collections import defaultdict
    d = defaultdict(str)
    prt_bin_tree(root, d)
    for k, v in sorted(d.items()):
        print(v)

Output result:

   1
  2  3
4   5  6
 7    8