[Python] Error in gui programming in python using tkinter

#!/usr/bin/python
# -*- coding: iso-8859-1 -*-

import Tkinter

class simpleapp_tk(Tkinter.Tk):
def __init__(self,parent):
Tkinter.Tk.__init__(self,parent)
self.parent=parent
def initialize(self):
self.grid()

self.entry=Tkinter.Entry(self)
self.entry.grid(column=0,row=0,sticky='EW')
self.entry.bind("<Return>",self.OnPressEnter)

button=Tkinter.Button(self,test="Post it!",command=self.OnButtonClick)
button.grid(column=1,row=0)

label=Tkinter.Label(self,anchor="w",fg="white",bg="blue")
label=grid(column=0,row=1,columnspan=2,sticky='EW')

self.grid_columnconfigure(0,weight=1)

def OnButtonClick(self):
print "you clicked the button!"

def OnPressEnter(self,event):
print "you pressed enter!"

if __name__=="__main__":
app=simpleapp_tk(None)
app.title('poster')
app.mainloop()


I wrote this program so as to make a box to input text and a button, but it shows nothing apart from a window. Where is the error?

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