def _make_test_folds(self, X, y=None, labels=None):
if self.shuffle:
rng = check_random_state(self.random_state)
else:
rng = self.random_state
y = np.asarray(y)
n_samples = y.shape[0]
unique_y, y_inversed = np.unique(y, return_inverse=True)
y_counts = bincount(y_inversed)
min_labels = np.min(y_counts)
if np.all(self.n_folds > y_counts):
raise ValueError("All the n_labels for individual classes"
" are less than %d folds."
% (self.n_folds))
if self.n_folds > min_labels:
warnings.warn(("The least populated class in y has only %d"
" members, which is too few. The minimum"
" number of labels for any class cannot"
" be less than n_folds=%d."
% (min_labels, self.n_folds)), Warning)
# pre-assign each sample to a test fold index using individual KFold
# splitting strategies for each class so as to respect the balance of
# classes
# NOTE: Passing the data corresponding to ith class say X[y==class_i]
# will break when the data is not 100% stratifiable for all classes.
# So we pass np.zeroes(max(c, n_folds)) as data to the KFold
per_cls_cvs = [
KFold(self.n_folds, shuffle=self.shuffle,
random_state=rng).split(np.zeros(max(count, self.n_folds)))
for count in y_counts]
test_folds = np.zeros(n_samples, dtype=np.int)
for test_fold_indices, per_cls_splits in enumerate(zip(*per_cls_cvs)):
for cls, (_, test_split) in zip(unique_y, per_cls_splits):
cls_test_folds = test_folds[y == cls]
# the test split can be too big because we used
# KFold(...).split(X[:max(c, n_folds)]) when data is not 100%
# stratifiable for all the classes
# (we use a warning instead of raising an exception)
# If this is the case, let's trim it:
test_split = test_split[test_split < len(cls_test_folds)]
cls_test_folds[test_split] = test_fold_indices
test_folds[y == cls] = cls_test_folds
return test_folds
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