[MRG+1] Fix message formatting in exception (#8319)

MMeketon · lesteve · commit a0db45db0e86 · 2017-02-21T10:03:38.000+01:00
when the missing class label is a string.
diff --git a/sklearn/utils/class_weight.py b/sklearn/utils/class_weight.py
@@ -66,7 +66,7 @@ def compute_class_weight(class_weight, classes, y):
         for c in class_weight:
             i = np.searchsorted(classes, c)
             if i >= len(classes) or classes[i] != c:
-                raise ValueError("Class label %d not present." % c)
+                raise ValueError("Class label {} not present.".format(c))
             else:
                 weight[i] = class_weight[c]
 
diff --git a/sklearn/utils/tests/test_class_weight.py b/sklearn/utils/tests/test_class_weight.py
@@ -31,6 +31,12 @@ def test_compute_class_weight_not_present():
     classes = np.arange(4)
     y = np.asarray([0, 0, 0, 1, 1, 2])
     assert_raises(ValueError, compute_class_weight, "balanced", classes, y)
+    # Fix exception in error message formatting when missing label is a string
+    # https://github.com/scikit-learn/scikit-learn/issues/8312
+    assert_raise_message(ValueError,
+                         'Class label label_not_present not present',
+                         compute_class_weight,
+                         {'label_not_present': 1.}, classes, y)
     # Raise error when y has items not in classes
     classes = np.arange(2)
     assert_raises(ValueError, compute_class_weight, "balanced", classes, y)
