Assume groups will have more than n memebers, I want to take every nth row from each group. I looked at https://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.core.groupby.GroupBy.nth.html but that only takes one row from each group.

For example:

    import pandas as pd
    x = pd.DataFrame.from_dict({'a': [1, 1, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3], 'b': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]})


    a   b
0   1   1
1   1   2

2   2   3
3   2   4
4   2   5

5   3   6
6   3   7
7   3   8
8   3   9
9   3  10
10  3  11
11  3  12

And if we are keeping every 2nd row:

    a   b
1   1   2

3   2   4

6   3   7
8   3   9
10  3  11