Sklearn: Handle the special case when all labels are the same

tests/classifiers/test_sklearn.py

@@ -0,0 +1,40 @@
+#  MIPLearn: Extensible Framework for Learning-Enhanced Mixed-Integer Optimization
+#  Copyright (C) 2020, UChicago Argonne, LLC. All rights reserved.
+#  Released under the modified BSD license. See COPYING.md for more details.
+
+import numpy as np
+from numpy.testing import assert_array_equal
+from sklearn.neighbors import KNeighborsClassifier
+
+from miplearn import ScikitLearnClassifier
+
+
+def test_constant_prediction():
+    x_train = np.array(
+        [
+            [0.0, 1.0],
+            [1.0, 0.0],
+        ]
+    )
+    y_train = np.array(
+        [
+            [True, False],
+            [True, False],
+        ]
+    )
+    clf = ScikitLearnClassifier(
+        KNeighborsClassifier(
+            n_neighbors=1,
+        )
+    )
+    clf.fit(x_train, y_train)
+    proba = clf.predict_proba(x_train)
+    assert_array_equal(
+        proba,
+        np.array(
+            [
+                [1.0, 0.0],
+                [1.0, 0.0],
+            ]
+        ),
+    )