code: def init_app(self, app):
          
          _state._register_app = self.original_register_app  # Restore Celery app registration function.
          if not hasattr(app, 'extensions'):
              app.extensions = dict()
          if 'celery' in app.extensions:
              raise ValueError('Already registered extension CELERY.')
          app.extensions['celery'] = _CeleryState(self, app)
  
          # Instantiate celery and read config.
          super(Celery, self).__init__(app.import_name, broker=app.config['CELERY_BROKER_URL'])
  
          # Set result backend default.
          if 'CELERY_RESULT_BACKEND' in app.config:
              self._preconf['CELERY_RESULT_BACKEND'] = app.config['CELERY_RESULT_BACKEND']
  
          self.conf.update(app.config)
          task_base = self.Task
  
          # Add Flask app context to celery instance.
          class ContextTask(task_base):
              def __call__(self, *_args, **_kwargs):
                  with app.app_context():
                      return task_base.__call__(self, *_args, **_kwargs)
          setattr(ContextTask, 'abstract', True)
          setattr(self, 'Task', ContextTask)
docs: Actual method to read celery settings from app configuration and initialize the celery instance.
  
          :param app: Flask application instance.