def categories(self):
        """
        Return the list of levels for an enum (categorical) column.

        This function can only be applied to single-column categorical frame.
        """
        if self.ncols != 1:
            raise H2OValueError("This operation only applies to a single factor column")
        if self.types[self.names[0]] != "enum":
            raise H2OValueError("Input is not a factor. This operation only applies to a single factor column")
        return self.levels()[0]