why "import wx" doesn't work?

[monkey]

I just learn to make a blank windows frame with python and wxpython. I found
the statment "import wx" cannot work as the original "from wxPython.wx
import *". I see in the readme file of wxpython that if I install it as the
default one, I can use "import wx" instead of the long one. What is wrong?
The code pasted below:

import wx   # the default is "from wxPython.wx import *", I change it and it
just can't work.

class MyApp(wxApp):
    def OnInit(self):
        frame = wxFrame(NULL, -1, "Hello from wxPython")
        frame.Show(true)
        self.SetTopWindow(frame)
        return true

app = MyApp(0)
app.MainLoop()