code: def get_forced_variation(self, experiment_key, user_id):
      
  
      if not self.is_valid:
        self.logger.error(enums.Errors.INVALID_DATAFILE.format('get_forced_variation'))
        return None
  
      if not validator.is_non_empty_string(experiment_key):
        self.logger.error(enums.Errors.INVALID_INPUT_ERROR.format('experiment_key'))
        return None
  
      if not isinstance(user_id, string_types):
        self.logger.error(enums.Errors.INVALID_INPUT_ERROR.format('user_id'))
        return None
  
      forced_variation = self.config.get_forced_variation(experiment_key, user_id)
      return forced_variation.key if forced_variation else None
docs: Gets the forced variation for a given user and experiment.
  
      Args:
        experiment_key: A string key identifying the experiment.
        user_id: The user ID.
  
      Returns:
        The forced variation key. None if no forced variation key.