title: Python : How to set id of button
body: <p>I am using <code>python-2.7</code> and <code>kivy-1.9.0</code>. Can someone tell me how to set <code>id</code> of button?
  I am trying to set id<code>test</code> of button using this code.</p>
  <pre><code>btn1 = Button(text=&quot;Close&quot;,id=&quot;test&quot;)
  </code></pre>
  <p>But it gives error <code>'Alert' object has no attribute 'test'</code></p>
  <h1>test.py</h1>
  <pre><code>from kivy.uix.screenmanager import Screen
  from kivy.app import App
  from kivy.lang import Builder
  from kivy.uix.popup import Popup
  from kivy.uix.label import Label
  from kivy.uix.button import Button
  from kivy.core.window import Window
  from kivy.uix.boxlayout import BoxLayout
  from kivy.properties import ObjectProperty
  
  Window.size = (500, 150)
  
  
  class Alert(Popup):
  
      def __init__(self, title, text):
          super(Alert, self).__init__()
  
          box = BoxLayout(orientation='vertical', padding=(5))
          box.add_widget(Label(text=text))
          btn1 = Button(text=&quot;Close&quot;,id=&quot;test&quot;)
          box.add_widget(btn1)
  
          self.title = title
          self.title_size = 30
          self.title_align = 'center'
          self.content = box
          self.size_hint = (None, None)
          self.size = (300, 200)
          self.auto_dismiss = False
  
          self.open()
          self.test.background_color = [0, 0, 1, 0.5]
  
  
  class Test(App):
      def build(self):
          Alert(title='yeah!', text='inputs are invalid')
          return
  
  
  if __name__ == '__main__':
      Test().run()
  </code></pre>