A test for [groupby](https://gist.github.com/mrocklin/5618992)

test_groupby.py

def test_groupby():
    names = ['Alice', 'Bob', 'Charlie', 'Dan', 'Edith', 'Frank']
    assert groupby(len, names) == {3: ['Bob', 'Dan'],
                                   5: ['Alice', 'Edith', 'Frank'],
                                   7: ['Charlie']}

    even = lambda x: x % 2 == 0
    assert groupby(even, range(10)) == {False: [1, 3, 5, 7, 9],
                                        True:  [0, 2, 4, 6, 8]}