code: def _set_categories(self, categories, fastpath=False):
          
  
          if fastpath:
              new_dtype = CategoricalDtype._from_fastpath(categories,
                                                          self.ordered)
          else:
              new_dtype = CategoricalDtype(categories, ordered=self.ordered)
          if (not fastpath and self.dtype.categories is not None and
                  len(new_dtype.categories) != len(self.dtype.categories)):
              raise ValueError("new categories need to have the same number of "
                               "items than the old categories!")
  
          self._dtype = new_dtype
docs: Sets new categories inplace
  
          Parameters
          ----------
          fastpath : bool, default False
             Don't perform validation of the categories for uniqueness or nulls
  
          Examples
          --------
          >>> c = pd.Categorical(['a', 'b'])
          >>> c
          [a, b]
          Categories (2, object): [a, b]
  
          >>> c._set_categories(pd.Index(['a', 'c']))
          >>> c
          [a, c]
          Categories (2, object): [a, c]