pandas add column to groupby dataframe

Question

I have this simple dataframe df:

df = pd.DataFrame({\'c\':[1,1,1,2,2,2,2],\'type\':[\'m\',\'n\',\'o\',\'m\',\'m\',\'n\',\'n\']})

Answer 1

Another solution with transform len:

df['size'] = df.groupby('c')['type'].transform(len)
print df
   c type size
0  1    m    3
1  1    n    3
2  1    o    3
3  2    m    4
4  2    m    4
5  2    n    4
6  2    n    4

Another solution with Series.map and Series.value_counts:

df['size'] = df['c'].map(df['c'].value_counts())
print (df)
   c type  size
0  1    m     3
1  1    n     3
2  1    o     3
3  2    m     4
4  2    m     4
5  2    n     4
6  2    n     4