code: def init_app(self, app):
          
          if not hasattr(app, 'extensions'):
              app.extensions = {}
          config = app.extensions.setdefault('whooshee', {})
          # mapping that caches whoosheers to their indexes; used by `get_or_create_index`
          config['whoosheers_indexes'] = {}
          # store a reference to self whoosheers; this way, even whoosheers created after init_app
          # was called will be found
          config['whoosheers'] = self.whoosheers
          config['index_path_root'] = app.config.get('WHOOSHEE_DIR', '') or 'whooshee'
          config['writer_timeout'] = app.config.get('WHOOSHEE_WRITER_TIMEOUT', 2)
          config['search_string_min_len'] = app.config.get('WHOOSHEE_MIN_STRING_LEN', 3)
          config['memory_storage'] = app.config.get("WHOOSHEE_MEMORY_STORAGE", False)
          config['enable_indexing'] = app.config.get('WHOOSHEE_ENABLE_INDEXING', True)
  
          if app.config.get('WHOOSHE_MIN_STRING_LEN', None) is not None:
              warnings.warn(WhoosheeDeprecationWarning("The config key WHOOSHE_MIN_STRING_LEN has been renamed to WHOOSHEE_MIN_STRING_LEN. The mispelled config key is deprecated and will be removed in upcoming releases. Change it to WHOOSHEE_MIN_STRING_LEN to suppress this warning"))
              config['search_string_min_len'] = app.config.get('WHOOSHE_MIN_STRING_LEN')
  
          if not os.path.exists(config['index_path_root']):
              os.makedirs(config['index_path_root'])
docs: Initialize the extension. It will create the `index_path_root`
          directory upon initalization but it will **not** create the index.
          Please use :meth:`reindex` for this.
  
          :param app: The application instance for which the extension should
                      be initialized.