Using self.xxxx as a default parameter - Python

Question

I'm trying to simplify one of my homework problems and make the code a little better. What I'm working with is a binary search tree. Right now I have a function in my Tree() class that finds all the elements and puts them into a list.

tree = Tree() #insert a bunch of items into tree 

then I use my makeList() function to take all the nodes from the tree and puts them in a list. To call the makeList() function, I do tree.makeList(tree.root). To me this seems a little repetitive. I'm already calling the tree object with tree.so the tree.root is just a waste of a little typing.

Right now the makeList function is:

    def makeList(self, aNode):         if aNode is None:             return []         return [aNode.data] + self.makeList(aNode.lChild) + self.makeList(aNode.rChild) 

I would like to make the aNode input a default parameter such as aNode = self.root (which does not work) that way I could run the function with this, tree.makeList().

First question is, why doesn't that work?
Second question is, is there a way that it can work? As you can see the makeList() function is recursive so I cannot define anything at the beginning of the function or I get an infinite loop.

EDIT Here is all the code as requested:

class Node(object):     def __init__(self, data):         self.data = data         self.lChild = None         self.rChild = None  class Tree(object):     def __init__(self):         self.root = None      def __str__(self):         current = self.root      def isEmpty(self):         if self.root == None:             return True         else:             return False      def insert (self, item):         newNode = Node (item)         current = self.root         parent = self.root          if self.root == None:             self.root = newNode         else:             while current != None:                 parent = current                 if item < current.data:                     current = current.lChild                 else:                     current = current.rChild              if item < parent.data:                 parent.lChild = newNode             else:                 parent.rChild = newNode      def inOrder(self, aNode):         if aNode != None:             self.inOrder(aNode.lChild)             print aNode.data             self.inOrder(aNode.rChild)      def makeList(self, aNode):         if aNode is None:             return []         return [aNode.data] + self.makeList(aNode.lChild) + self.makeList(aNode.rChild)       def isSimilar(self, n, m):         nList = self.makeList(n.root)         mList = self.makeList(m.root)          print mList == nList  

Answer 1

larsmans answered your first question

For your second question, can you simply look before you leap to avoid recursion?

def makeList(self, aNode=None):     if aNode is None:         aNode = self.root     treeaslist = [aNode.data]     if aNode.lChild:         treeaslist.extend(self.makeList(aNode.lChild))     if aNode.rChild:         treeaslist.extend(self.makeList(aNode.rChild))     return treeaslist