Add new column to dataframe that is another column's values from the month before based repeating datetime index with other columns as identifiers

If I have this df called feature_df:

Each row represents a particular "cohort" of mortgage loan groups. I want to select the Wac from each row and create a new column called lagged_WAC which is filled with Wac values from the month prior, based on the datetime index called y_m. Additionally, each lagged Wac must correspond with the Vintage and cluster column values for that row. That is why there are repeats for each date. Each row contains data for each mortgage cohort (Vintage, Coupon, and bondsec_code) at that time. The dataset starts at February 2019 though, so there wouldn't be any "previous months values" for any of those rows. How can I do this?

Here is a more reproducible example with just the index and Wac column:

              Wac
y_m 
2019-04-01  3.4283
2019-04-01  4.1123
2019-04-01  4.4760
2019-04-01  3.9430
2019-04-01  4.5702
... ...
2022-06-01  2.2441
2022-06-01  4.5625
2022-06-01  5.6446
2022-06-01  4.0584
2022-06-01  3.0412

I have tried implementing this code to generate a copy dataframe and then lagged values by a month, then merging back with the original, but I'm not sure how to check that the Wac_y values returned with the new merged df are correct:

df1 = feature_df.copy().reset_index()
df1['new_date'] = df1['y_m'] + pd.DateOffset(months=-1)
df1 = df1[['Wac', 'new_date']]
feature_df.merge(df1, left_index=True, right_on = 'new_date')

For example, there are values for 2019-01-01 which I don't know where they come from since the original dataframe doesn't have data for that month, and the shape goes from 20,712 rows to 12,297,442 rows

I can't test it because I don't have representative data, but from what I see you could try something like this.

df['lagged_WAC'] = df.groupby('cluster', sort=False, as_index=False)['Wac'].shift(1)

If each month has unique clusters for each Wac value, you can groupby cluster and then shift the each row in a group by one to the past. If you need to groupby more than one column you need to pass a list to the groupby like df.groupby(['Vintage', 'cluster']).

Made a little example dataset to show you what I'm thinking of. This is my input:

        Month       Wac cluster
0  2017-04-01  2.271980     car
1  2017-04-01  2.586608     bus
2  2017-04-01  2.071009   plane
3  2017-04-01  2.102676    boat
4  2017-05-01  2.222338     car
5  2017-05-01  2.617924     bus
6  2017-05-01  2.377280   plane
7  2017-05-01  2.150043    boat
8  2017-06-01  2.203132     car
9  2017-06-01  2.072133     bus
10 2017-06-01  2.223499   plane
11 2017-06-01  2.253821    boat
12 2017-07-01  2.228020     car
13 2017-07-01  2.717485     bus
14 2017-07-01  2.446508   plane
15 2017-07-01  2.607244    boat
16 2017-08-01  2.116647     car
17 2017-08-01  2.820238     bus
18 2017-08-01  2.186937   plane
19 2017-08-01  2.827701    boat

df['lagged_WAC'] = df.groupby('cluster', sort=False,as_index=False)['Wac'].shift(1)
print(df)

Output:

        Month       Wac cluster  lagged_WAC
0  2017-04-01  2.271980     car         NaN
1  2017-04-01  2.586608     bus         NaN
2  2017-04-01  2.071009   plane         NaN
3  2017-04-01  2.102676    boat         NaN
4  2017-05-01  2.222338     car    2.271980
5  2017-05-01  2.617924     bus    2.586608
6  2017-05-01  2.377280   plane    2.071009
7  2017-05-01  2.150043    boat    2.102676
8  2017-06-01  2.203132     car    2.222338
9  2017-06-01  2.072133     bus    2.617924
10 2017-06-01  2.223499   plane    2.377280
11 2017-06-01  2.253821    boat    2.150043
12 2017-07-01  2.228020     car    2.203132
13 2017-07-01  2.717485     bus    2.072133
14 2017-07-01  2.446508   plane    2.223499
15 2017-07-01  2.607244    boat    2.253821
16 2017-08-01  2.116647     car    2.228020
17 2017-08-01  2.820238     bus    2.717485
18 2017-08-01  2.186937   plane    2.446508
19 2017-08-01  2.827701    boat    2.607244

the first month has only Nan because there is no earlier month. Each car in that df has now the value for car in the previous month, each boat for boat in the previous month and so on.