def col_values(self, col, value_render_option='FORMATTED_VALUE'):
        """Returns a list of all values in column `col`.

        Empty cells in this list will be rendered as :const:`None`.

        :param col: Column number.
        :type col: int
        :param value_render_option: (optional) Determines how values should be
                                    rendered in the the output. See
                                    `ValueRenderOption`_ in the Sheets API.
        :type value_render_option: str

        .. _ValueRenderOption: https://developers.google.com/sheets/api/reference/rest/v4/ValueRenderOption

        """

        start_label = rowcol_to_a1(1, col)
        range_label = '%s!%s:%s' % (self.title, start_label, start_label[:-1])

        data = self.spreadsheet.values_get(
            range_label,
            params={
                'valueRenderOption': value_render_option,
                'majorDimension': 'COLUMNS'
            }
        )

        try:
            return data['values'][0]
        except KeyError:
            return []