wxPython - Expand() method in wx.TreeCtrl

import wx 
class MyTree(wx.TreeCtrl): 
    def __init__(self, parent, id, pos, size, style): 
        wx.TreeCtrl.__init__(self, parent, id, pos, size, style) 
class TreePanel(wx.Panel): 
    def __init__(self, parent): 
        wx.Panel.__init__(self, parent) 
        
        # create tree control in window 
        self.tree = MyTree(self, wx.ID_ANY, wx.DefaultPosition, 
                           wx.DefaultSize, wx.TR_HAS_BUTTONS) 
        
        # CREATE TREE ROOT 
        self.root = self.tree.AddRoot('root') 
        self.tree.SetPyData(self.root, ('key', 'value')) 
        # add item to root 
        item = self.tree.AppendItem(self.root, "Item") 
        item2 = self.tree.AppendItem(self.root, "Item") 
        # expand root 
        self.tree.Expand(self.root) 
        
        sizer = wx.BoxSizer(wx.VERTICAL) 
        sizer.Add(self.tree, 0, wx.EXPAND) 
        self.SetSizer(sizer) 
class MainFrame(wx.Frame): 
    def __init__(self): 
        wx.Frame.__init__(self, parent = None, title ='TreeCtrl Demo') 
        panel = TreePanel(self) 
        self.Show() 
if __name__ == '__main__': 
    app = wx.App(redirect = False) 
    frame = MainFrame() 
    app.MainLoop()