Sample n rows from each group in pandas

Question

In the dataframe below, I am doing groupby on three fields: 'Subject', 'Rep' and 'yval'.

import pandas as pd 
yval = [[1]*30 + [2]*20 + [1]*20 + [2]*30 ]
yval = reduce(lambda x,y: x+y, yval)
df = pd.DataFrame({'yval': yval , 'xval':np.random.randn(100)})
df['Subject'] = ['S01'] * 50 + ['S02'] * 50
l = [[x] * 10 for x in range(3)] + [[x] * 10 for x in range(2)] + [[x] * 10 for x in range(2)] + [[x] * 10 for x in range(3)]
l = reduce(lambda x,y: x+y,l)
df['Rep'] = l
df


for k, t in df.groupby(['Subject', 'yval', 'Rep']):
    print k 


('S01', 1, 0)
('S01', 1, 1)
('S01', 1, 2)
('S01', 2, 0)
('S01', 2, 1)
('S02', 1, 0)
('S02', 1, 1)
('S02', 2, 0)
('S02', 2, 1)
('S02', 2, 2)

I am trying to find a way to select n rows from group each. In this example, assuming n = 2, we might get the following result. If n=4, I expect everything (the entire dataframe).

('S01', 1, 0)
('S01', 1, 2)
('S01', 2, 0)
('S01', 2, 1)
('S02', 1, 0)
('S02', 1, 1)
('S02', 2, 1)
('S02', 2, 2)

Answer 1

The previous answer selects n groups, whereas OP wants to select n rows from each group. Then it should be done as

ix = np.hstack([np.random.choice(v, n, replace=False) for v in gps.groups.values()])

where gps = df.groupby(['Subject', 'yval', 'Rep']).

Then df.iloc(ix) will give n rows selected randomly from each group.