pydata/pandas — pull request #6569 by hayd (2014-03-07 17:50:43+00:00)
Title: ENH/BUG groupby nth now filters, works with DataFrames

fixes #5552

partial for #5264
```
In [101]: df = pd.DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=['A', 'B'])

In [102]: g = df.groupby('A')

In [103]: g.nth(0)
Out[103]:
A B
0 1 NaN
2 5 6

In [104]: g.nth(1)
Out[104]:
A B
1 1 4

In [105]: g.nth(-1)
Out[105]:
A B
1 1 4
2 5 6

In [106]: g.nth(0, dropna='any') # old behaviour-like
Out[106]:
B
A
1 4
5 6

In [107]: g.nth(1, dropna='any') # old behaviour-like
Out[107]:
B
A
1 NaN
5 NaN
```