<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def OnExpandAll(self):
 """ expand all nodes """

        root = self.tree.GetRootItem()
        fn = self.tree.Expand
        self.traverse(root, fn)
        self.tree.Expand(root)