可以使用pd.MultiIndex.from_product
这样的一些变化：
In [24]: x = pd.date_range('2019-01-01', '2019-04-01', freq='MS')
In [25]: y = ['a', 'b', 'c']
In [26]: index = pd.MultiIndex.from_product([x, y])
In [27]: for ix in index:
...: print(ix)
...:
...:
...:
(Timestamp('2019-01-01 00:00:00', freq='MS'), 'a')
(Timestamp('2019-01-01 00:00:00', freq='MS'), 'b')
(Timestamp('2019-01-01 00:00:00', freq='MS'), 'c')
(Timestamp('2019-02-01 00:00:00', freq='MS'), 'a')
(Timestamp('2019-02-01 00:00:00', freq='MS'), 'b')
(Timestamp('2019-02-01 00:00:00', freq='MS'), 'c')
(Timestamp('2019-03-01 00:00:00', freq='MS'), 'a')
(Timestamp('2019-03-01 00:00:00', freq='MS'), 'b')
(Timestamp('2019-03-01 00:00:00', freq='MS'), 'c')
(Timestamp('2019-04-01 00:00:00', freq='MS'), 'a')
(Timestamp('2019-04-01 00:00:00', freq='MS'), 'b')
(Timestamp('2019-04-01 00:00:00', freq='MS'), 'c')
