Merging a pandas groupby result back into DataFrame

Question

I have a DataFrame that looks like this...

   idn value  
0  ID1    25
1  ID1    30
2  ID2    30
3  ID2    50

I want to add another column to t

Answer 1

Use the transform method on a groupby object:

In [5]: df['maxval'] = df.groupby(by=['idn']).transform('max')

In [6]: df
Out[6]: 
   idn  value  maxval
0  ID1     25      30
1  ID1     30      30
2  ID2     30      50
3  ID2     50      50