Merge 2fabc0d into 9790bc6

scikit-learn · Dec 17, 2015 · 19d662d · 19d662d
2 parents 9790bc6 + 2fabc0d
commit 19d662d
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diff --git a/sklearn/preprocessing/label.py b/sklearn/preprocessing/label.py
@@ -9,6 +9,7 @@
 from collections import defaultdict
 import itertools
 import array
+import warnings
 
 import numpy as np
 import scipy.sparse as sp
@@ -295,6 +296,9 @@ def fit(self, y):
 
         self.sparse_input_ = sp.issparse(y)
         self.classes_ = unique_labels(y)
+        if len(self.classes_) == 1:
+                warnings.warn("Only one label in y and this label will be "
+                              "regarded as negative one.")
         return self
 
     def transform(self, y):

diff --git a/sklearn/preprocessing/tests/test_label.py b/sklearn/preprocessing/tests/test_label.py
@@ -12,6 +12,7 @@
 from sklearn.utils.testing import assert_array_equal
 from sklearn.utils.testing import assert_equal
 from sklearn.utils.testing import assert_raises
+from sklearn.utils.testing import assert_warns_message
 from sklearn.utils.testing import ignore_warnings
 
 from sklearn.preprocessing.label import LabelBinarizer
@@ -43,6 +44,8 @@ def test_label_binarizer():
     assert_array_equal(lb.classes_, ["pos"])
     assert_array_equal(expected, got)
     assert_array_equal(lb.inverse_transform(got), inp)
+    w = "Only one label in y and this label will be regarded as negative one."
+    assert_warns_message(UserWarning, w, lb.fit, inp)
 
     # two-class case
     inp = ["neg", "pos", "pos", "neg"]