code: def test_ondoubleclick(self, fopen):
      mbt = self.mbt
  
      with mock.patch("os.path.exists", return_value=False):
          mbt.OnDoubleClick()
          fopen.assert_not_called()
  
      with mock.patch("os.path.exists", return_value=True):
          mbt.OnDoubleClick()
          fopen.assert_called_once_with(fname)
  
imports: from idlelib import browser
  from test.support import requires
  import unittest
  from unittest import mock
  from idlelib.idle_test.mock_idle import Func
  from idlelib.util import py_extensions
  from collections import deque
  import os.path
  import pyclbr
  from tkinter import Tk
  from idlelib.tree import TreeNode
label_desc: Use image node_id 8 for calling the ModuleBrowserTreeItemTest obj's underlying member method code with example usage: obj.test_ondoubleclick(fopen) without return types