Add extra column as the cumulative time difference

Question

How to add an extra column that is the cumulative value of the time differences for each course? For example, the initial table is:

 id_A       course     we         


        

Answer 1

Use groupby, transform, and .iloc:

df['ts_A'] = pd.to_datetime(df.ts_A)
df['cum_delta_sec'] = (df.groupby('id_A')['ts_A']
                         .transform(lambda x: (x - x.iloc[0]).dt.total_seconds()))

Output:

  id_A       course  weight                ts_A       value  cum_delta_sec
0  id1       cotton     3.5 2017-04-27 01:35:30  150.000000              0
1  id1       cotton     3.5 2017-04-27 01:36:00  416.666667             30
2  id1       cotton     3.5 2017-04-27 01:36:30  700.000000             60
3  id1       cotton     3.5 2017-04-27 01:37:00  950.000000             90
4  id2  cotton blue     5.0 2017-04-27 02:35:30  150.000000              0
5  id2  cotton blue     5.0 2017-04-27 02:36:00  450.000000             30
6  id2  cotton blue     5.0 2017-04-27 02:36:30  520.666667             60
7  id2  cotton blue     5.0 2017-04-27 02:37:00  610.000000             90

In the group, subtract current value from the first value and use .dt accessor to convert to seconds.