Mine

class Node(object):    '''A single node in the tree; contains a value, pointers to the left and    right subtrees, as well as how many nodes there are in the subtree rooted    at this node.'''        def __init__(self, value):        '''Create a new node containing value with no children.'''        self.value = value        self.left = None        self.right = None        self.size = 1        # You should not add any more attributes to this class.        class TreeList(object):    '''A partial implementation of a list using a binary tree.'''        def __init__(self):        '''Create an empty list.'''        self.root = None        # You should not add any more attributes to this class.            def set_root(self, new_root):        '''Set the root of the tree to be new_root.'''        self.root = new_root            def get_root(self):        '''Return a pointer to the root node of this tree.'''        return self.root        def add_left_or_right(self, r_node, value, isleft):        if isleft:            r_node.left = Node(value)        else:            r_node.right = Node(value)        def find_node(self, r_node, add, value, index, insert):        i = add + r_node.size - 1                if r_node.right is not None:            i -= r_node.right.size                if i + 1 == index:            if r_node.right is None:                if insert:                    self.add_left_or_right(r_node, value, False)                else:                    return r_node            else:                add += 1                r_node.size += 1                return self.find_node(r_node.right, add, value, index, insert)        elif i == index:            if r_node.left is None:                if insert:                    self.add_left_or_right(r_node, value, True)                else:                    return r_node            else:                r_node.size += 1                return self.find_node(r_node.left, add, value, index, insert)        else:            if i > index:                r_node.size += 1                return self.find_node(r_node.left, add, value, index, insert)            else:                add += 1                r_node.size += 1                return self.find_node(r_node.right, add, value, index, insert)            def get(self, index):        '''Return element index of the list.'''        if index < 0:            raise Exception("Index is less than zero.")        elif self.root is not None:            if index > self.root.size - 1:                raise Exception("Index exceeds number of elements in list.")        elif self.root is None and index > 0:            raise Exception("Index exceeds number of elements in list.")                current = self.root        _left = current.left        _right = current.right                if _left is not None:            if index == _left.size:                needed = current            elif index < _left.size: #Working with left side.                needed = self.find_node(_left, 0, 0, index, False)            else: #Working with right side.                addition = current.size - current.right.size                needed = self.find_node(_right, addition, 0, index, False)        elif _right is not None: #Working with right side.            if index == 0:                needed = current            else:                needed = self.find_node(_right, 1, 0, index, False)        return needed.value                def insert(self, index, value):        '''Insert value at position index, shifting everything else to the        right.  Always inserts value to be a leaf.'''                if index < 0:            raise Exception("Index is less than zero.")        elif self.root is not None:            if index > self.root.size:                raise Exception("Index exceeds number of elements in list.")        elif self.root is None and index > 0:            raise Exception("Index exceeds number of elements in list.")                if self.root is None:            self.root = Node(value)            return                    current = self.root        _left = current.left        _right = current.right                if index == 0:            while current is not None:                current.size += 1                before = current                current = current.left            before.left = Node(value)            return                     if index == current.size:            while current is not None:                current.size += 1                before = current                current = current.right            before.right = Node(value)            return        if _left is not None:            if index <= _left.size: #Working with left side.                self.find_node(_left, 0, value, index, True)            else: #Working with right side.                addition = current.size - current.right.size                current.size += 1                self.find_node(_right, addition, value, index, True)        elif _right is not None: #Working with right side.            self.find_node(_right, 1, value, index, True)                current.size += 1    def size(self):        '''How many elements are in the list.'''        if self.root is None:            return 0        return self.root.size    if __name__ == "__main__":    my_list = TreeList()    my_list.insert(0, 1)    my_list.insert(1, 2) # The "list" is now [1,2]    print my_list.get(1) # Should print 2    my_list.insert(1, 3) # The "list" is now [1,3,2]    print my_list.get(1) # Should print 3    try:       my_list.insert(4, 7) # Should raise an exception, my_list should be unchanged.    except Exception, e:        print 'Exception handled.'        print my_list.get(1) # Should print 3