beginning gui programming, could use some help

[billySometimes]

I've decided to mess around with Tkinter and import os a bit. I'm running ubuntu 8.10, if that helps any.

Here is what my program was supposed to do:
have two simple buttons, one labeled 'on' and one labeled 'off'. I wanted the on button to launch nautilus and the off button to quit the program and therefore close nautilus, just as it would if I launched nautilus from the terminal.

What it is actually doing:
As soon as i run the program, nautilus launches, and my window with the buttons don't pop up until after I have closed nautilus. Why is this?

Here is my code:

from Tkinter import *       
import os
class Application(Frame):              
     def __init__(self, master=None):
        Frame.__init__(self, master)   
        self.grid()                    
        self.createWidgets()

     def createWidgets(self):
        self.onButton = Button ( self, text="on",
            command=os.system('nautilus' ))        
        self.onButton.grid(row=0, column=0)         
        self.offButton = Button ( self, text="off",
            command=self.quit )
        self.offButton.grid(row=0, column=1)

app = Application()                    
app.master.title("My Application") 
app.mainloop()

Thanks in advance for your help!

[Gribouillis]

It's because os.system waits that the subprocess is finished. You should not use os.system anymore in python. Use the subprocess module

import subprocess
child_process = subprocess.Popen("nautilus", shell=True)

This won't wait for the subprocess. See the documentation of the subprocess module for more complex usage.