title: "self.data" usage in python
body: <p>Hello I have a question about attribute usage in Python. I learned that in function definition we can assign some new attributes for objects for example: self! However, when try to use that I got an error which is "... instance has no attribute 'data'</p>
  
  <pre><code>class Lazy:
      def __call__(self, num):
              if num is None:
                      return self.data
              # It changes the current object in-place,
              # by reassigning the self attribute.
              else:
                      self.data += num
  </code></pre>
  
  <p>This is my little code.
  I'm very new at this. I couldn't figured out what is wrong.
  Thank you very much.</p>