title: formatting string in python with class members
body: <p>Consider this code:</p>
  
  <pre><code>class Foo:
    def geta(self):
      self.a = 'lie'
      return 'this is {self.a}'.format(?)
  </code></pre>
  
  <p>What should I write instead of the question mark so that the string will be formatted correctly?</p>