You can use pandas transform() method for within group aggregations like “OVER(partition by …)” in SQL:
import pandas as pd
import numpy as np
#create dataframe with sample data
df = pd.DataFrame({'group':['A','A','A','B','B','B'],'value':[1,2,3,4,5,6]})
#calculate AVG(value) OVER (PARTITION BY group)
df['mean_value'] = df.groupby('group').value.transform(np.mean)
df:
group value mean_value
A 1 2
A 2 2
A 3 2
B 4 5
B 5 5
B 6 5