title: Pycharm warning: must implement all abstract methods
body: <p><strong>Code</strong></p>
  
  <pre><code>class A(object):
      def a(self):
          raise NotImplementedError
  
  class B(A):
      def a(self):
          return 7
  
  class C(B):
      pass
  </code></pre>
  
  <p><strong>Why does Pycharm complain?</strong></p>
  
  <blockquote>
    <p>Problem synopsis      Class C must implement all abstract methods </p>
  </blockquote>