title: Tkinter is opening new windows when running a function in a thread
body: <p>Hi all I am using python 2.7.15 and tkinter. It is a simple GUI with some buttons. Once a button is pressed I need to start a function in a thread (I do not need to open any new windows).</p>
  
  <p>What is happening is for each thread a new GUI copy of the program is opened. Is there any way to start a function (that does some calculations) without popping up a new copy of the Tkinter gui?</p>
  
  <p>I am making a thread like this:</p>
  
  <pre><code>thread = Process(target=functionName, args=(arg1, arg2))
  thread.start()
  thread.join()
  </code></pre>
  
  <p>EDIT: here is some code to reproduce. As you can see, all I am interested in below "sample" is to run one function. Not to clone the whole program.</p>
  
  <pre><code>from Tkinter import *
  from multiprocessing import Process
  
  window = Tk()
  
  window.title("Test threadinng")
  
  window.geometry('400x400')
  
  
  def threadFunction():
      sys.exit()
  
  def start():
      thread1 = Process(target=threadFunction)
      thread2 = Process(target=threadFunction)
      thread1.start()
      thread2.start()
      thread1.join()
      thread2.join()
  
  btn = Button(window, text="Click Me", command=start, args=())
  
  btn.grid(column=1, row=1)
  
  window.mainloop()
  </code></pre>
  
  <p>Thank you.</p>