How to Coalesce datetime values from 3 columns into a single column in a pandas dataframe?

Question

I have a dataframe with 3 date columns in datetime format:

CLIENT_ID	DATE_BEGIN	DATE_START	DATE_REGISTERED
1	2020-01-01	2020-01-01	2020-01-01
2	2020-01-02	2020-02-01	2020-01-01
3	NaN	2020-05-01	2020-04-01
4	2020-01-01	2020-01-01	NaN

How do I create (coalesce) a new column with the earliest datetime for each row resulting in an ACTUAL_START_DATE

CLIENT_ID	DATE_BEGIN	DATE_START	DATE_REGISTERED	ACTUAL_START_DATE
1	2020-01-01	2020-01-01	2020-01-01	2020-01-01
2	2020-01-02	2020-02-01	2020-01-01	2020-01-01
3	NaN	2020-05-01	2020-04-01	2020-04-01
4	2020-01-01	2020-01-02	NaN	2020-01-01

some sort of variation with bfill?

Answer 1

You are right, a mix of bfill and ffill on the axis columns should do it:

df.assign(ACTUAL_START_DATE = df.filter(like='DATE')
                                .bfill(axis=1)
                                .ffill(axis=1)
                                .min(axis=1)
        )
 
   CLIENT_ID  DATE_BEGIN  DATE_START DATE_REGISTERED ACTUAL_START_DATE
0          1  2020-01-01  2020-01-01      2020-01-01        2020-01-01
1          2  2020-01-02  2020-02-01      2020-01-01        2020-01-01
2          3         NaN  2020-05-01      2020-04-01        2020-04-01
3          4  2020-01-01  2020-01-01             NaN        2020-01-01