Find the categorical columns.

        Parameters
        ----------
        X : pandas.DataFrame or dask.DataFrame
        y : ignored

        Returns
        -------
        self

def fit(self, X, y=None):
        """Find the categorical columns.

        Parameters
        ----------
        X : pandas.DataFrame or dask.DataFrame
        y : ignored

        Returns
        -------
        self
        """
        X = self._check_array(X)

        if self.categories is not None:
            # some basic validation
            columns = pd.Index(self.categories)
            categories = self.categories

        elif isinstance(X, pd.DataFrame):
            columns, categories = self._fit(X)
        else:
            columns, categories = self._fit_dask(X)

        self.columns_ = columns
        self.categories_ = categories
        return self