Import 2 wx windows from each other

Newbie Help
1

Hi.
Imagine that we have 1.py and 2.py as a main window and a sub window, and we want to create a button to navigate between them.
For an explanation of the idea see the code:
1.py (the main window):

import wx

class app(wx.Frame):
	def __init__(self, parent):
		super(app,self).__init__(parent,-1, title= 'example app')
		p = wx.Panel(self,-1)
		self.Center()
		self.btn = wx.Button(p,-1, 'go')
		self.btn.Bind(wx.EVT_BUTTON, self.onBtn)
		self.Show()

	def onBtn(self, event):
		# I need the code to import the 2.py's class.

app = wx.App()
app(None)
app.MainLoop()

2.py (the sub window):

import wx

class appTwo(wx.Frame):
	def __init__(self, parent):
		super(appTwo,self).__init__(parent,-1, title= 'example app')
		p = wx.Panel(self,-1)
		self.Center()
		self.btn = wx.Button(p,-1, 'back')
		self.btn.Bind(wx.EVT_BUTTON, self.onBtn)
		self.Show()

	def onBtn(self, event):
		# I need the code to import the 1.py's class.

app = wx.App()
app2(None)
app.MainLoop()

so, what’s the code shuld we put in the onBtn def?