[Python] Using self.xxxx as a default parameter for recursive function - Python

I'm trying to simplify my work and fine-tune the code. 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: why doesn't that work?
Second question: is there a way that it can work (for this kind of recursive function)? As you can see the makeList() function is recursive so I cannot unconditionally initialize as any constant value at the beginning of the function or else 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

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