code: def get_user_by_key(app, key):
      
      if not hasattr(app, 'example_user_db'):
          app.example_user_db = {}
  
      if key in app.example_user_db:
          return app.example_user_db[key]
      return None
docs: This method is only an example! Replace it with a real user database.
  
      :param str key: the public key the user belongs to