what does 0 mean in MyApp(0)

[Alex]

I'm looking at a tutorial with the code below

from wxPython.wx import *

class MyApp(wxApp):
    def OnInit(self):
        frame = wxFrame(NULL, -1, "winApp", size = (800,640))
        frame.Show(true)
        self.SetTopWindow(frame)
        return true

app = MyApp(0)
app.MainLoop()

Everything is explained nicely except the zero parameter in MyApp(0).
Anybody knows what that zero refers to? 

Alex