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

You can chain the diff method with cumsum:

# convert ts_A to datetime type
df.ts_A = pd.to_datetime(df.ts_A)

# convert ts_A to seconds, group by id and then use transform to calculate the cumulative difference
df['cum_delta_sec'] = df.ts_A.astype(int).div(10**9).groupby(df.id_A).transform(lambda x: x.diff().fillna(0).cumsum())
df