问个问题：app = exApp(0)，给这个 0 有什么用啊

CODE:

from wxPython.wx import *
from wxPython.html import *
import os,sys

class exHtmlWindow(wxHtmlWindow):
   def __init__(self, parent, id, frame):
      wxHtmlWindow.__init__(self,parent,id)

class exHtmlPanel(wxPanel):
   def __init__(self, parent, id, frame):
      wxPanel.__init__(self,parent,-1)

      self.html = exHtmlWindow(self, -1, frame)

      self.box = wxBoxSizer(wxVERTICAL)
      self.box.Add(self.html, 1, wxGROW)

      self.SetSizer(self.box)
      self.SetAutoLayout(true)

class exFrame (wxFrame):
   def __init__(self, parent, ID, title):
      wxFrame.__init__(self,parent,ID,title,wxDefaultPosition,wxSize(600,750))
      panel = exHtmlPanel(self, -1, self)

class exApp(wxApp):
   def OnInit(self):
      frame = exFrame(NULL, -1, "Example Browser")
      frame.Show(true)
      self.SetTopWindow(frame)
      return true


app = exApp(0)
app.MainLoop()