code: def _expand(indexer, retain_categories):
      """
      Expand Index to all categories.
      Applies to categorical index, and used
      in _computations_pivot_wider for scenarios where
      names_expand and/or index_expand is True.
      Categories are preserved where possible.
      If `retain_categories` is False, a fastpath is taken
      to generate all possible combinations.
  
      Returns an Index.
      """
      if indexer.nlevels > 1:
          names = indexer.names
          if not retain_categories:
              indexer = pd.MultiIndex.from_product(
                  indexer.levels, names=names
              )
          else:
              indexer = [
                  indexer.get_level_values(n)
                  for n in range(indexer.nlevels)
              ]
              indexer = [
                  pd.Categorical(
                      values=arr.categories,
                      categories=arr.categories,
                      ordered=arr.ordered,
                  )
                  if is_categorical_dtype(arr)
                  else arr.unique()
                  for arr in indexer
              ]
              indexer = pd.MultiIndex.from_product(indexer, names=names)
  
      else:
          if not retain_categories:
              indexer = indexer.categories
          else:
              indexer = pd.Categorical(
                  values=indexer.categories,
                  categories=indexer.categories,
                  ordered=indexer.ordered,
              )
      return indexer
  
imports: import re
  import warnings
  from collections import defaultdict
  from itertools import chain, zip_longest
  from typing import Callable, Optional, Pattern, Union
  import numpy
  import pandas
  import pandas_flavor
  from pandas.api.types import is_categorical_dtype, is_extension_array_dtype, is_list_like
  from pandas.core.dtypes.concat import concat_compat
  from janitor.functions.utils import _computations_expand_grid, get_index_labels
  from janitor.utils import check, refactored_function
label_desc: Use image node_id 15 for calling a global function with example usage: _expand(indexer, retain_categories) and returns: indexer