pandas: how do I select first row in each GROUP BY group?

Question

Basically the same as Select first row in each GROUP BY group? only in pandas.

df = pd.DataFrame({'A' : ['foo', 'foo', 'foo', 'foo', 'bar', 'bar', 'bar', 'bar'],                 'B' : ['3', '1', '2', '4','2', '4', '1', '3'],                     }) 

Sorting looks promising:

df.sort('B')       A  B 1  foo  1 6  bar  1 2  foo  2 4  bar  2 0  foo  3 7  bar  3 3  foo  4 5  bar  4 

But then first won't give the desired result... df.groupby('A').first()

     B A      bar  2 foo  3 

Answer 1

Generally if you want your data sorted in a groupby but it's not one of the columns which are going to be grouped on then it's better to sort the df prior to performing groupby:

In [5]: df.sort_values('B').groupby('A').first()  Out[5]:      B A      bar  1 foo  1