code: def _normalize_doc_type_to_list(doc_type):
      # Ensure to have a list of index
      if doc_type is None:
          searchable_doc_types = []
      elif isinstance(doc_type, str):
          searchable_doc_types = [doc_type]
      elif isinstance(doc_type, list):
          searchable_doc_types = doc_type
      else:
          # Is it the correct exception to use ?
          raise ValueError("Invalid param 'index'")
  
      return searchable_doc_types
  
imports: from __future__ import annotations
  import fnmatch
  import json
  from elasticsearch import Elasticsearch
  from elasticsearch.exceptions import NotFoundError
  from .utilities import MissingIndexException, get_random_id, query_params
label_desc: Use image node_id 19 for calling the FakeElasticsearch obj's underlying member method code with example usage: obj._normalize_doc_type_to_list(doc_type) and returns: searchable_doc_types