code: def _prepare_paths(self):
          
          if config.BASE in self.paths:
              for path in [path for path in self.paths if path != config.BASE]:
                  if config.BASE_VARIABLE in self.paths[path]:
                      self.paths[path] = \
                          self.paths[path].replace(config.BASE_VARIABLE,
                                                   self.paths[config.BASE])
          LOGGER.debug('Prepared paths: %r', self.paths)
docs: Set the value of {{base}} in paths if the base path is set in the
          configuration.
  
          :raises: ValueError