title: Attribute created in one method doesn't exist in other method
body: <p>Here I have an attribute 'a', which is defined in first class method and should be changed in second.
  When calling them in order, this message appears:</p>
  
  <blockquote>
    <p>AttributeError: 'Class' object has no attribute 'a'</p>
  </blockquote>
  
  <p>The only way I've found - define 'a' again in second method, but in real code it has long inheritance and app will be messed.
  Why doesn't it work? Isn't <em>self.a</em> equal to <em>Class.a</em>?</p>
  
  <pre><code>class Class(object):
      def method_1(self):
          self.a = 1
      def method_2(self):
          self.a += 1
  
  Class().method_1()
  Class().method_2()
  </code></pre>