如何根据 python 中的另一列填充一列？

Question

I have a df that look like this:

ID       Test Done      Test Action    Test Date
1234     Happy Test     Decline        2021-11-30
1234     None           Decline        None
1235     Sad Test       Decline        2022-03-24
1235     None           Decline        2022-03-04
1235     None           Decline        2022-03-04
1236     Lonely Test    Decline        2022-05-06
1236     Lonely Test    Decline        2022-05-06
1236     Lonely Test    Decline        2022-05-06

I am trying to populate all the None or empty fields in Test Done that are associated with an ID number. So I want my df to look like....

ID       Test Done      Test Action    Test Date
1234     Happy Test     Decline        2021-11-30
1234     Happy Test     Decline        None
1235     Sad Test       Decline        2022-03-24
1235     Sad Test       Decline        2022-03-04
1235     Sad Test       Decline        2022-03-04
1236     Lonely Test    Decline        2022-05-06
1236     Lonely Test    Decline        2022-05-06
1236     Lonely Test    Decline        2022-05-06

I am not sure how to go about this. From what I searched on the web, I did not find anything that related to this specific question I had or find any functions that could answer my question.

Edit:

I want to populate just the None values with the first value that is shown in Test Done. So for instance in the example the first value is Happy Test with ID 1234, I wand the None value to be Happy Test, the same goes for Sad Test with 1235 ID. If an ID already has a Test Done populated then we can skip that. Hope this makes sense.

Answer 1

Use a groupby with ffill() .

data = {'id': [1234, 1234, 1235, 1235, 1235, 1236, 1236, 1236],
        'test': ['Happy Test', 
                  None, 
                 'Sad Test', 
                  None, 
                  None, 
                 'Lonely Test', 
                 'Lonely Test', 
                 'Lonely Test']
       }

df = pd.DataFrame(data)

df['test'] = df.groupby('id')['test'].ffill()

Output:

     id         test
0  1234   Happy Test
1  1234   Happy Test
2  1235     Sad Test
3  1235     Sad Test
4  1235     Sad Test
5  1236  Lonely Test
6  1236  Lonely Test
7  1236  Lonely Test