Fix issue 112: EpochScoring flaky when y_test is None.

skorch/callbacks.py

@@ -272,7 +272,7 @@ class EpochScoring(ScoringBase):
       This is called on y before it is passed to scoring.
 
     """
-    # pylint: disable=unused-argument
+    # pylint: disable=unused-argument,arguments-differ
     def on_epoch_end(
             self,
             net,
@@ -291,7 +291,10 @@ def on_epoch_end(
         if X_test is None:
             return
 
-        y_test = self.target_extractor(y_test)
+        if y_test is not None:
+            # We allow y_test to be None but the scoring function has
+            # to be able to deal with it (i.e. called without y_test).
+            y_test = self.target_extractor(y_test)
         current_score = self._scoring(net, X_test, y_test)
         history.record(self.name_, current_score)
 

skorch/tests/test_callbacks.py

@@ -272,6 +272,30 @@ def test_target_extractor_is_called(
 
         assert extractor.call_count == 2
 
+    def test_without_target_data_works(
+            self, net_cls, module_cls, scoring_cls, data,
+    ):
+        def myscore(_, X, y=None):
+            assert y is None
+            return np.mean(X)
+
+        def mysplit(X, y):
+            # set y_valid to None
+            return X, X, y, None
+
+        X, y = data
+        net = net_cls(
+            module=module_cls,
+            callbacks=[scoring_cls(myscore)],
+            train_split=mysplit,
+            max_epochs=2,
+        )
+        net.fit(X, y)
+
+        expected = np.mean(X)
+        loss = net.history[:, 'myscore']
+        assert np.allclose(loss, expected)
+
 
 class TestBatchScoring:
     @pytest.fixture