title: display a QMessageBox PyQT when a different combobox /list box item is selected
body: <p>I have a combo box <code>cbLayer</code> and a function <code>do_stuff</code> of the following form:</p>
  
  <pre><code>def do_stuff(item_selected_from_cbLayer):
      new_list = []
      # do stuff based on item_selected_from_combobox and put the items in new_list
      return new_list
  </code></pre>
  
  <p>How can I get a <code>QMessageBox</code> to pop up whenever a different item is selected in the following form:</p>
  
  <pre><code>QMessageBox.warning(self, "items: ", do_stuff(cb_selected_item))
  </code></pre>