Python shows wx window,but menu bar is not displayed. Why?

Question

I've installed wxPython on my mac and tried this program

import wx

class MainWindow(wx.Frame):
    def __init__(self, parent, title):
        wx.Frame.__init__(self, parent, title=title, size=(200,100))
        self.control = wx.TextCtrl(self, style=wx.TE_MULTILINE)
        self.CreateStatusBar() # A Statusbar in the bottom of the window

        # Setting up the menu.
        filemenu= wx.Menu()

        # wx.ID_ABOUT and wx.ID_EXIT are standard IDs provided by wxWidgets.
        filemenu.Append(wx.ID_ABOUT, "&About"," Information about this program")
        filemenu.AppendSeparator()
        filemenu.Append(wx.ID_EXIT,"E&xit"," Terminate the program")

        # Creating the menubar.
        menuBar = wx.MenuBar()
        menuBar.Append(filemenu,"&File") # Adding the "filemenu" to the MenuBar
        self.SetMenuBar(menuBar)  # Adding the MenuBar to the Frame content.
        self.Show(True)

app = wx.App(False)
frame = MainWindow(None, "Sample editor")
app.MainLoop()

I started python2.7 to run it, it prompts a window, but it doesn't show any menubar, out of my expectation.

Did I miss anything in my program, or I need any extra python/wx configurations?