Python Pandas Choosing Random Sample of Groups from Groupby

问题

What is the best way to get a random sample of the elements of a groupby? As I understand it, a groupby is just an iterable over groups.

The standard way I would do this for an iterable, if I wanted to select N = 200 elements is:

rand = random.sample(data, N)  

If you attempt the above where data is a 'grouped' the elements of the resultant list are tuples for some reason.

I found the below example for randomly selecting the elements of a single key groupby, however this does not work with a multi-key groupby. From, How to access pandas groupby dataframe by key

create groupby object

grouped = df.groupby('some_key')

pick N dataframes and grab their indices

sampled_df_i = random.sample(grouped.indices, N)

grab the groups using the groupby object 'get_group' method

df_list = map(lambda df_i: grouped.get_group(df_i),sampled_df_i)

optionally - turn it all back into a single dataframe object

sampled_df = pd.concat(df_list, axis=0, join='outer')

回答1:

You can take a randoms sample of the unique values of df.some_key.unique(), use that to slice the df and finally groupby on the resultant:

In [337]:

df = pd.DataFrame({'some_key': [0,1,2,3,0,1,2,3,0,1,2,3],
                   'val':      [1,2,3,4,1,5,1,5,1,6,7,8]})
In [338]:

print df[df.some_key.isin(random.sample(df.some_key.unique(),2))].groupby('some_key').mean()
               val
some_key          
0         1.000000
2         3.666667

If there are more than one groupby keys:

In [358]:

df = pd.DataFrame({'some_key1':[0,1,2,3,0,1,2,3,0,1,2,3],
                   'some_key2':[0,0,0,0,1,1,1,1,2,2,2,2],
                   'val':      [1,2,3,4,1,5,1,5,1,6,7,8]})
In [359]:

gby = df.groupby(['some_key1', 'some_key2'])
In [360]:

print gby.mean().ix[random.sample(gby.indices.keys(),2)]
                     val
some_key1 some_key2     
1         1            5
3         2            8

But if you are just going to get the values of each group, you don't even need to groubpy, MultiIndex will do:

In [372]:

idx = random.sample(set(pd.MultiIndex.from_product((df.some_key1, df.some_key2)).tolist()),
                    2)
print df.set_index(['some_key1', 'some_key2']).ix[idx]
                     val
some_key1 some_key2     
2         0            3
3         1            5

来源：https://stackoverflow.com/questions/32340604/python-pandas-choosing-random-sample-of-groups-from-groupby