Pandas group by selected dates

Question

I have a dataframe that is very similar to this dataframe:

index	date	month
0	2019-12-1	12
1	2020-03-1	3
2	2020-07-1	7
3	2021-02-1	2
4	2021-09-1	9

And i want to combine all dates that are closest to a set of months. The months need to be normalized like this:

Months	Normalized month
3, 4, 5	4
6, 7, 8, 9	8
1, 2, 10, 11, 12	12

So the output will be:

index	date	month
0	2019-12-1	12
1	2020-04-1	4
2	2020-08-1	8
3	2020-12-1	12
4	2021-08-1	8

Answer 1

You can iterate through the DataFrame and use replace to change the dates.

import pandas as pd 

df = pd.DataFrame(data={'date': ["2019-12-1", "2020-03-1", "2020-07-1", "2021-02-1", "2021-09-1"], 
                        'month': [12,3,7,2,9]})
for index, row in df.iterrows():
    if (row['month'] in [3,4,5]):
        df['month'][index] = 4
        df["date"][index]  = df["date"][0].replace(df["date"][0][5:7],"04")
    elif (row['month'] in [6,7,8,9]):
        df['month'][index] = 8
        df["date"][index]  = df["date"][0].replace(df["date"][0][5:7],"08")
    else:
        df['month'][index] = 12
        df["date"][index]  = df["date"][0].replace(df["date"][0][5:7],"12")