title: adding function to kivy object
body: <p>I am trying to check the active status of a checkbox with the following:</p>
  <p>checkbox.py</p>
  <pre><code>from kivy.uix.checkbox import CheckBox
  
  class CheckBox(CheckBox):
      def checkbox_click(self, value):
          if value:
              print('is active', value)
          else:
              print('is inactive', value)
  </code></pre>
  <p>and then in the kv file</p>
  <pre><code>CheckBox:
       id: decisionsyes
       text: 'yes'
       group: 'decisions'
       state:'down'
       on_active: self.checkbox_click(self, self.active)
  </code></pre>
  <p>But I get the error:</p>
  <blockquote>
  <p>AttributeError: 'CheckBox' object has no attribute 'checkbox_click'</p>
  </blockquote>
  <p>Any idea how I could do that please?</p>