title: Python kivy (kivyMD) how to fetch values of MDDropdownMenu
body: <p>I am trying to fetch the selection of a <code>MDDropwdownMenu</code> inside my kv file I am using a <code>MDRaisedButton</code> by the following code</p>
  
  <pre><code> MDRaisedButton:
                              id: select_warning_image_Button
                              text: "Menu labels"                          
                              opposite_colors:    True
                              elevation_normal:    0
                              on_release: MDDropdownMenu(items=app.menu_labels, width_mult=4).open(self)
  </code></pre>
  
  <p>In my <code>main.py</code> (see below) I have a class called <code>MainApp</code> which inherits from <code>App</code>. Inside this class I have created the attribute variable <code>menu_labels</code>. I want to use the function <code>change_variable</code> to set the value of the variable <code>VARIABLE</code> to the value of the menu. But I cannot seem to be able to use self.change_variable(…). Is there a value to fire a function when a specific value in the dropdown is selected?</p>
  
  <pre><code># main.py
  import …
  
  
  class MainApp(App):
      VARIABLE = ""
  
      menu_labels = [
          {"viewclass": "MDMenuItem",
           "text": "Label1",
           "on_release": self.change_variable("Label1")},
          {"viewclass": "MDMenuItem",
           "text": "Label2",
           "on_release": self.change_variable("Label2")},
      ]
  
      def change_variable(self, label):
          self.VARIABLE = label
  </code></pre>