How to parse a tree in python?

Question

I have an expression as follow: ( root (AA ( CC) (DD) ) (BB (CC) (DD) ) )

I want to parse this expression using recursion and I created a tree class but I'm stuck after this man.

This looks this the following tree.

                   root
                     |
                ____________
              AA           BB
              |             |  
       __________         ___________
      CC      DD          CC      DD

The output should look like this:

Root -> AA BB
AA -> CC DD
BB -> CC DD

My tree class look like the following:

 class tree_parsing(object):

     def __init__(self, element=None):
         self.element = element
         self.children = []

Basically I want to store the children in a list member variable of the class. Can someone help figure this problem out? Thank You.

It would be helpful to know exactly where you're starting from. Is the task to parse the text string "( root (AA ( CC) (DD) ) (BB (CC) (DD) ) )"? — Arthur Dent
– Arthur Dent, Commented Nov 21, 2013 at 14:10
Sounds like you want to implement breath first search, wikipedia gives some hints including a rough implementation: en.wikipedia.org/wiki/Breadth-first_search — Jan Henke
– Jan Henke, Commented Nov 21, 2013 at 14:14
OK. I'd suggest proceeding as follows: Scan the string left to right. Parse the left parenthesis to mean "start a new tree node class, possibly as a child of the one I'm working on now." Ignore spaces and parse non-parentheses as portions of a name that will be completed by a parenthesis (either open or closed). Closed parenthesis will have a meaning that will become clear as you go, I think. Hopefully, that will give you a start. — Arthur Dent
– Arthur Dent, Commented Nov 21, 2013 at 14:17
BFS should be the last step: the output. But for building the tree, he'll certainly need to write some lexer and parser - which ain't that straightforward. — Paulo Bu
– Paulo Bu, Commented Nov 21, 2013 at 14:20

score 1 · Accepted Answer · 2013-11-21 15:33:03Z

1

You can do something like this:

#!python
# -*- coding: utf-8 -*-

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

def dump_tree(node):
  if node.left or node.right:
    left = 'None' if node.left is None else node.left.name
    right = 'None' if node.right is None else node.right.name
    print('{0} -> {1} {2}'.format(node.name, left, right))

  if node.left:
    dump_tree(node.left)

  if node.right:
    dump_tree(node.right)


Root = Node(
  'Root', Node('AA', Node('CC'), Node('DD')),
          Node('BB', Node('CC'), Node('DD')),
)

dump_tree(Root)

prints:

$ python example.py
Root -> AA BB
AA -> CC DD
BB -> CC DD
$

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