You just need to use apply
on the groupby
object. I modified your example data to make this a little more clear:
import pandas
from io import StringIO
csv = StringIO("""index,A,B
0,1,0.0
1,1,3.0
2,1,6.0
3,2,0.0
4,2,5.0
5,2,7.0""")
df = pandas.read_csv(csv, index_col="index")
groups = df.groupby(by=['A'])
print(groups.apply(lambda g: g[g['B'] == g['B'].max()]))
Which prints:
A B
A index
1 2 1 6
2 4 2 7