python - Get group id back into pandas dataframe

Question

For dataframe

In [2]: df = pd.DataFrame({'Name': ['foo', 'bar'] * 3,
   ...:                    'Rank': np.random.randint(0,3,6),
   ...:                    'Val': np.random.rand(6)})
   ...: df
Out[2]: 
  Name  Rank       Val
0  foo     0  0.299397
1  bar     0  0.909228
2  foo     0  0.517700
3  bar     0  0.929863
4  foo     1  0.209324
5  bar     2  0.381515

I'm interested in grouping by Name and Rank and possibly getting aggregate values

In [3]: group = df.groupby(['Name', 'Rank'])
In [4]: agg = group.agg(sum)
In [5]: agg
Out[5]: 
                Val
Name Rank          
bar  0     1.839091
     2     0.381515
foo  0     0.817097
     1     0.209324

But I would like to get a field in the original df that contains the group number for that row, like

In [13]: df['Group_id'] = [2, 0, 2, 0, 3, 1]
In [14]: df
Out[14]: 
  Name  Rank       Val  Group_id
0  foo     0  0.299397         2
1  bar     0  0.909228         0
2  foo     0  0.517700         2
3  bar     0  0.929863         0
4  foo     1  0.209324         3
5  bar     2  0.381515         1

Is there a good way to do this in pandas?

I can get it with python,

In [16]: from itertools import count
In [17]: c = count()
In [22]: group.transform(lambda x: c.next())
Out[22]: 
   Val
0    2
1    0
2    2
3    0
4    3
5    1

but it's pretty slow on a large dataframe, so I figured there may be a better built in pandas way to do this.

Answer 1

A lot of handy things are stored in the DataFrameGroupBy.grouper object. For example:

>>> df = pd.DataFrame({'Name': ['foo', 'bar'] * 3,
                   'Rank': np.random.randint(0,3,6),
                   'Val': np.random.rand(6)})
>>> grouped = df.groupby(["Name", "Rank"])
>>> grouped.grouper.
grouped.grouper.agg_series        grouped.grouper.indices
grouped.grouper.aggregate         grouped.grouper.labels
grouped.grouper.apply             grouped.grouper.levels
grouped.grouper.axis              grouped.grouper.names
grouped.grouper.compressed        grouped.grouper.ngroups
grouped.grouper.get_group_levels  grouped.grouper.nkeys
grouped.grouper.get_iterator      grouped.grouper.result_index
grouped.grouper.group_info        grouped.grouper.shape
grouped.grouper.group_keys        grouped.grouper.size
grouped.grouper.groupings         grouped.grouper.sort
grouped.grouper.groups            

and so:

>>> df["GroupId"] = df.groupby(["Name", "Rank"]).grouper.group_info[0]
>>> df
  Name  Rank       Val  GroupId
0  foo     0  0.302482        2
1  bar     0  0.375193        0
2  foo     2  0.965763        4
3  bar     2  0.166417        1
4  foo     1  0.495124        3
5  bar     2  0.728776        1

There may be a nicer alias for for grouper.group_info[0] lurking around somewhere, but this should work, anyway.