Python – Adding grouped mode as additional column in original dataset

So I have data similar to this:

import pandas as pd
df = pd.DataFrame({'Order ID':[555,556,557,558,559,560,561,562,563,564,565,566],
                   'State':["MA","MA","MA","MA","MA","MA","CT","CT","CT","CT","CT","CT"],
                   'County':["Essex","Essex","Essex","Worcester","Worcester","Worcester","Bristol","Bristol","Bristol","Hartford","Hartford","Hartford"],
                   'AP':[50,50,75,100,100,125,150,150,175,200,200,225]})

but I need to add a column that shows the mode of AP grouped by State and County. I can get the mode this way:

(df.groupby(['State', 'County']).AP.agg(Mode = (lambda x: x.value_counts().index[0])).reset_index().round(0))

I’m just not sure how I can get that data added to the original data so that it looks like this:

>Solution :

Use GroupBy.transform for new column:

df['Mode'] = (df.groupby(['State', 'County']).AP
                .transform(lambda x: x.value_counts().index[0]))

Or Series.mode:

df['Mode'] = df.groupby(['State', 'County']).AP.transform(lambda x: x.mode().iat[0])