def __call__(self, df):
    assert all((isinstance(col, _DataFrameColumn) for col in self.cols))
    return self.df._yield_pdf().sort_values(by=[col.index for col in self.cols], ascending=self.ascending)
def __call__(self, df):
    assert all((isinstance(col, _DataFrameColumn) for col in self.cols))
    return self.df._yield_pdf().sort_values(by=[col.index for col in self.cols], ascending=self.ascending)