Make Pandas groupby act similarly to itertools groupby

First you can identify which elements in the Grp column differ from the previous and get the cumulative sum to form the groups you need:

In [9]:
    diff_to_previous = df.Grp != df.Grp.shift(1)
    diff_to_previous.cumsum()
Out[9]:

0    1
1    2
2    2
3    3
4    3
5    4
6    4
7    5
8    5
9    6

So you can then do

df.groupby(diff_to_previous.cumsum()) 

to get the desired groupby object