How to implement a binary search tree in Python?

class Node: 
    rChild,lChild,data = None,None,None

This is wrong - it makes your variables class variables - that is, every instance of Node uses the same values (changing rChild of any node changes it for all nodes!). This is clearly not what you want; try

class Node: 
    def __init__(self, key):
        self.rChild = None
        self.lChild = None
        self.data = key

now each node has its own set of variables. The same applies to your definition of Tree,

class Tree:
    root,size = None,0    # <- lose this line!
    def __init__(self):
        self.root = None
        self.size = 0

Further, each class should be a "new-style" class derived from the "object" class and should chain back to object.__init__():

class Node(object): 
    def __init__(self, data, rChild=None, lChild=None):
        super(Node,self).__init__()
        self.data   = data
        self.rChild = rChild
        self.lChild = lChild

class Tree(object):
    def __init__(self):
        super(Tree,self).__init__()
        self.root = None
        self.size = 0

Also, main() is indented too far - as shown, it is a method of Tree which is uncallable because it does not accept a self argument.

Also, you are modifying the object's data directly (t.root = Node(4)) which kind of destroys encapsulation (the whole point of having classes in the first place); you should be doing something more like

def main():
    t = Tree()
    t.add(4)    # <- let the tree create a data Node and insert it
    t.add(5)

Here is a quick example of a binary insert:

class Node:
    def __init__(self, val):
        self.l_child = None
        self.r_child = None
        self.data = val

def binary_insert(root, node):
    if root is None:
        root = node
    else:
        if root.data > node.data:
            if root.l_child is None:
                root.l_child = node
            else:
                binary_insert(root.l_child, node)
        else:
            if root.r_child is None:
                root.r_child = node
            else:
                binary_insert(root.r_child, node)

def in_order_print(root):
    if not root:
        return
    in_order_print(root.l_child)
    print root.data
    in_order_print(root.r_child)

def pre_order_print(root):
    if not root:
        return        
    print root.data
    pre_order_print(root.l_child)
    pre_order_print(root.r_child)    

---

r = Node(3)
binary_insert(r, Node(7))
binary_insert(r, Node(1))
binary_insert(r, Node(5))

---

     3
    / \
   1   7
      /
     5

---

print "in order:"
in_order_print(r)

print "pre order"
pre_order_print(r)

in order:
1
3
5
7
pre order
3
1
7
5