如何 select dataframe 中每个组的最旧记录？ 使用 python

Question

As an example, a dataframe that looks like this:

         date  price ticker  volume
0   2018-01-01  1.323     AI    2000
1   2018-01-02  1.525     AI    1500
2   2018-01-03  1.045     AI     500
3   2018-01-04  1.845     AI     600
4   2018-01-05  1.045     AI     500
5   2018-01-02  1.446    BOC     550
6   2018-01-03  2.110    BOC    3201
7   2018-01-04  2.150    BOC    5200
8   2018-01-05  2.810    BOC    1980
9   2018-01-03  5.199    CAT    2000
10  2018-01-06  4.980    CAT     450
11  2018-01-07  4.990    CAT    3000

I am going to ask a very basic question, please bear with me how can choose the first two ticker that has oldest date, to have a dataframe like below

         date  price ticker  volume
0   2018-01-01  1.323     AI    2000
1   2018-01-02  1.525     AI    1500
5   2018-01-01  1.446    BOC     550
6   2018-01-02  2.110    BOC    3201
9   2018-01-01  5.199    CAT    2000
10  2018-01-02  4.980    CAT     450

Answer 1

On Pandas, you can use groupby command to group values. Also, by using head command with groupby command, you can select first two values in the group. So, in your case, to group first two ticker, the command will be:

df.sort_values('date').groupby('ticker').head(2)