scikit-learn/scikit-learn — pull request #1497 by sdicker8 (2012-12-29 06:02:12+00:00)
Title: train_test_split: test_size default is None

If test_size is None and train_size is not None, test_size
is the complement of train_size. If test_size and train_size
are None, test_size = 0.25.
Docstring is updated.
Existing tests didn't seem to cover much of existing behaviour
and no updates were added. New behaviour was tested.