首先按条件值分组

Question

I have a pandas dataframe, like this:

ID	date	status
10	2022-01-01	0
10	2022-01-02	0
10	2022-01-03	1
10	2022-01-04	1
10	2022-01-05	1
23	2022-02-02	0
23	2022-02-03	0
23	2022-02-04	1
23	2022-02-05	1
23	2022-02-06	1

I would like to group per ID and the first date on status is equal 1.

Expected output:

ID	date	status	first_status
10	2022-01-03	1	2022-01-03
23	2022-02-03	1	2022-02-03

afteer this I will merge this new DF with previous DF. Final DF:

ID	date	status	first_status
10	2022-01-01	0	2022-01-03
10	2022-01-02	0	2022-01-03
10	2022-01-03	1	2022-01-03
10	2022-01-04	1	2022-01-03
10	2022-01-05	1	2022-01-03
23	2022-02-02	0	2022-02-04
23	2022-02-03	0	2022-02-04
23	2022-02-04	1	2022-02-04
23	2022-02-05	1	2022-02-04
23	2022-02-06	1	2022-02-04

I tried many ways to do this, but unsuccessful

Answer 1

Get the first date for status=1 for each ID. Then map each ID to the first date:

#convert to datetime if needed
df["date"] = pd.to_datetime(df["date"])

df["first_status"] = df["ID"].map(df[df["status"].eq(1)].groupby("ID")["date"].min())

>>> df
   ID       date  status first_status
0  10 2022-01-01       0   2022-01-03
1  10 2022-01-02       0   2022-01-03
2  10 2022-01-03       1   2022-01-03
3  10 2022-01-04       1   2022-01-03
4  10 2022-01-05       1   2022-01-03
5  23 2022-02-02       0   2022-02-04
6  23 2022-02-03       0   2022-02-04
7  23 2022-02-04       1   2022-02-04
8  23 2022-02-05       1   2022-02-04
9  23 2022-02-06       1   2022-02-04

Answer 2

You can filter the status 1 rows, and get the first (or min depending on the use case) per group, then merge to the orginal dataframe:

df2 = (df[df['status'].eq(1)]
       .groupby('ID', as_index=False)
       ['date'].first() # could also use "min()"
       .rename(columns={'date': 'first_status'})
      )

df.merge(df2, on='ID')

output:

   ID        date  status first_status
0  10  2022-01-01       0   2022-01-03
1  10  2022-01-02       0   2022-01-03
2  10  2022-01-03       1   2022-01-03
3  10  2022-01-04       1   2022-01-03
4  10  2022-01-05       1   2022-01-03
5  23  2022-02-02       0   2022-02-04
6  23  2022-02-03       0   2022-02-04
7  23  2022-02-04       1   2022-02-04
8  23  2022-02-05       1   2022-02-04
9  23  2022-02-06       1   2022-02-04

intermediate df2 :

   ID first_status
0  10   2022-01-03
1  23   2022-02-04