Another take at the IntTree using something like S-Expressions.

Node.py

'''Tree representing
       1
      /|\ 
     2 3 4 
    /\   |\
   5  6  9 11
      /\  \
     7  8  10
     
Not especially happy with this. Feels like I'm fighting the representation.
'''

# encoding for tree above as S-Expression
test_input = '1(2(5 6(7 8))3 4(9(10)11))'

class Node():
	def __init__(self, value):
		self.children = []
		self.value = value
	
	def __str__(self):
		return "%s - %s" % (self.value, str(self.children))
	
	def __repr__(self):
		return "(%r,%r)" % (self.value,self.children)

def parse_tree(field_gen):
	next_node = Node('')
	new_nodes = []
	node_completed = False
	while True:
		try:
			op = next(field_gen)[1]
			if op == '(':
				next_node.children = parse_tree(field_gen)
				new_nodes.append(next_node)
				next_node = Node('')
			elif op == ')':
				if next_node.value != '':
					new_nodes.append(next_node)
				return new_nodes
			elif op == ' ':
				if next_node.value != '':
					node_completed = True
					new_nodes.append(next_node)
			else:
				if node_completed:
					next_node = Node('')
				next_node.value += op
		except StopIteration:
			if next_node != None:
				new_nodes.append(next_node)
			return new_nodes
	
def write_tree(tree):
	result = str(tree.value)
	if len(tree.children) > 0:
		result += '('
		for i,c in enumerate(tree.children):
			if len(c.children) > 0:
				result += write_tree(c)  
			else:
				result += str(c.value)
				if i < (len(tree.children)-1):
					result += ' '
		result += ')'
	return result

def main():
	tree = parse_tree(enumerate(test_input))
	#print("%r" % (tree))
	test_output = write_tree(tree[0])
	print(test_output)
	again = parse_tree(enumerate(test_output))
	#print("%r" % (again))
	# now test reflexivity
	print(write_tree(again[0]))

#if __name__ == "__main__":
main()