title: Kivy: Changing screens in screen manager with an on_press event
body: <p>I would like to know how to change screens using an on_press event binded to a button, without using a KV file/KV language.</p>
  
  <p>I have read through the Kivy documentation, but have only been able to find solutions using a KV file.</p>
  
  <p>Example:</p>
  
  <p><code>on_press: root.manager.current = 'screen2'</code></p>
  
  <p>I can also change the screen in the main python file using:</p>
  
  <p><code>screenmanager.current = 'screen2'</code></p>
  
  <p>But I cant figure out how to achieve the same using a button.</p>