。新手。麻烦高手指教python的代码

求助。新手。麻烦高手指教python的代码
错误信息：Traceback (most recent call last):
  File "C:\Users\amazing\Desktop\C扩展\hope.py", line 3, in load
  file =open(filename.GetValue())
IOError: [Errno 22] invalid mode ('r') or filename: u''
代码如下：
import wx
def load(event):
  file =open(filename.GetValue())
  contents.SetValue(file.read())
  file.close()
def save(event):
  file =open(filename.GetValue(), 'w')
  file.write(contents.GetValue())
  file.close()
app=wx.App()
win=wx.Frame(None,title="simple editor",size=(410,335))
bkg=wx.Panel(win)
loadButton=wx.Button(bkg,label='Open')
loadButton.Bind(wx.EVT_BUTTON,load)
saveButton=wx.Button(bkg,label='save')
saveButton.Bind(wx.EVT_BUTTON,save)
filename=wx.TextCtrl(bkg)
contents=wx.TextCtrl(bkg,style=wx.TE_MULTILINE|wx.HSCROLL)
hbox=wx.BoxSizer()
hbox.Add(filename,proportion=1,flag=wx.EXPAND)
hbox.Add(loadButton,proportion=0,flag=wx.LEFT,border=5)
hbox.Add(saveButton,proportion=0,flag=wx.LEFT,border=5)
vbox=wx.BoxSizer(wx.VERTICAL)
vbox.Add(hbox,proportion=0,flag=wx.EXPAND|wx.ALL,border=5)
vbox.Add(contents,proportion=1,flag=wx.EXPAND|wx.LEFT|wx.BOTTOM|wx.RIGHT,border=5)
bkg.SetSizer(vbox)
win.Show()
app.MainLoop()

谁可以告诉我听。哪里错了~~

------解决方案--------------------
filename根本没初始化。。。