def _three_way_split(splitter: KFold,
X,
y: Optional = None,
groups: Optional = None) -> Generator:
"""A modified version of BaseCrossValidator.split().
Yields (K-2/1/1) train/val/test splits.
"""
X, y, groups = indexable(X, y, groups)
indices = np.arange(_num_samples(X))
test_masks_it = splitter._iter_test_masks(X, y, groups)
first_mask = last_mask = next(test_masks_it)
for test_mask in test_masks_it:
train_index = indices[np.logical_not(
np.logical_or(test_mask, last_mask))]
val_index = indices[last_mask]
test_index = indices[test_mask]
yield train_index, val_index, test_index
last_mask = test_mask
# last fold
test_mask = first_mask
train_index = indices[np.logical_not(
np.logical_or(test_mask, last_mask))]
val_index = indices[last_mask]
test_index = indices[test_mask]
yield train_index, val_index, test_index
