Issues about the time index(which will chage the type)
I found that if the time order is assending, the dataframe will change from pd.DataFrame to pd.Series automatically and vise versa.
 For example, df is the data i download from tushare and i set `df.set_index(['date'],inplace=True)`.
If the time index is assending, i cannot get `df['%Y-%m-%d']` like `df['2018-09-25']` because `type(df.loc['2018-09-25'])` is pd.Series. 
And if i change df into df[::-1], then the time order is decending and i can get `df['2018-09-25']` .What'more, the result of `type(df.loc['2018-09-25'])` is pd.DataFrame.

What should i do if i need the result of `type(df.loc['2018-09-25'])` is pd.DataFrame,no matter the time order is assending or decending.

