Get name of the column with last value in the row

I am doing cohort analysis and the dataset I’m using has 15 months as the name as columns with revenue and around 7k user_id rows.
I need to get a new column with the month when the user was last time active.

Result should be like this

I have tried few options but it didnt work

users.apply(pd.Series.last_valid_index)

>Solution :

Example

data = {'2021-01-01': {0: 3456, 1: None}, '2021-02-01': {0: None, 1: 8679}}
df = pd.DataFrame(data)

df

    2021-01-01  2021-02-01
0   3456.0      NaN
1   NaN         8679.0

Code

df.apply(lambda x: x.last_valid_index(), axis=1)

output:

0    2021-01-01
1    2021-02-01
dtype: object

full Code

df.assign(Last_month=df.apply(lambda x: x.last_valid_index(), axis=1))

result:

    2021-01-01  2021-02-01  Last_month
0   3456.0      NaN         2021-01-01
1   NaN         8679.0      2021-02-01