缺省值pd.isna pd.isnull pd.notna pd.notnull np.nan isnull = isna _isna_new(obj) _isna_old(obj) _isna = _isna_new _use_inf_as_na(key) _isna_ndarraylike(obj) _isna_ndarraylike_old(obj) notna(obj) notnull = notna array_equivalent(left