Pandas使用教程（八）
一、How to use the MultiIndex in pandas?
Question:如何select
stocks.reset_index()将index恢复默认
二、New change in 0.18.1
1.Create a datetime column from a DataFrame
2.Create a category column during file reading
三、New change in 0.19.0
1.Convert the data type of multiple columns at once
四、New change in 0.20.0
1.Apply multiple aggregations on a Series or DataFrame
the followings are new：
五、New change in 0.21.0
1.Aliases（别名）have been added for isnull and notnull
2.drop now accepts “index” and “columns” keywords
删除行：
删除列：
3.rename and reindex now accept “axis” keyword
especially:mapper can be a function:
4.Ordered categories must be specified independent of the data
