title: Kivy Widget ids access in __init__
body: <p>I have a simple custom widget defined in kv file:</p>
  
  <pre><code>&lt;CustomWidget&gt;:
      TextInput:
          id: text_input
  </code></pre>
  
  <p>And a class implementing it:</p>
  
  <pre><code>class CustomWidget(BoxLayout):
      def __init__(self, *args, **kwargs):
          super(CustomWidget, self).__init__(*args, **kwargs)
          self.ids['text_input'].bind(insert_text=numeric_filter)
  </code></pre>
  
  <p>This code fails, because <code>self.ids</code> is not populated at the point I'm trying to use it. Is there some callback or function of <code>kivy.uix.Widget</code> which I can override to perform first time setup my widgets?</p>