Apply vs transform on a group object

Question

Consider the following dataframe:

     A      B         C         D
0  foo    one  0.162003  0.087469
1  bar    one -1.156319 -1.526272
2  foo    two  0.8338         


        

Answer 1

tmp = df.groupby(['A'])['c'].transform('mean')

is like

tmp1 = df.groupby(['A']).agg({'c':'mean'})
tmp = df['A'].map(tmp1['c'])

or

tmp1 = df.groupby(['A'])['c'].mean()
tmp = df['A'].map(tmp1)