Statement orders

[Monu Agrawal]

Hi I am making a gui based tool. When user preses a perticular button I
am running a heavy command, before this I want to say user to wait with
a image showing infront of her.
My code is like:
	def loadData(self):
		top=Toplevel(self.parent)
		top.focus_set()
		self.parent.wm_title("Loading Data...")

		os.system('a heavy command')
		os.system('another heavy command)
		top.destroy()

Now when I apply it, it first runs the os.system commands then it shows
the top level.
I tried with putting sleep just before os.system() it didn't help.
Is some statement reordering going on, or I am missing something?