title: Tkinter second App window updating after the execution reaches last line
body: <p>I'm new to Tkinter and I'm facing issue of updating the listbox in second window.
  The issue is the listbox in secondary window get updated only after the execution reaches to last line and not in the middle.</p>
  <p>At First the secondary window was popping after the execution reached last line but used <strong>app.lift()</strong> function lift it but it's not updating</p>
  <p>Here is the short snippet of code :</p>
  <pre><code>class Demo:
      def m1(LB1):
          for i in range(10):
              #rest of code
              LB1.insert(END,i) 
  
  
  def f1():
      app2 = Tk()
      app2.title(&quot;PROCESSING&quot;)
      app2.geometry(&quot;350x50&quot;)
      app2.lift()
      LB1 = Listbox(app2, height=25, width=100)
      obj = Demo()
      obj.m1(LB1)
  
  
  def main():
      app = Tk()
      app.title(&quot;APP&quot;)
      #rest of code
      b1 = Button(app, text=&quot;start&quot;, height=1, width=80, command=f1)
  
  
  if __name__ == '__main__':
      main()
  </code></pre>
  <p>Thanks in advance!!</p>