[Fix] Raise AssertError when eval_res is a null dict

mmcv/runner/hooks/evaluation.py

@@ -358,6 +358,9 @@ class EvalHook(Hook):
         """
         eval_res = self.dataloader.dataset.evaluate(
             results, logger=runner.logger, **self.eval_kwargs)
+
+        assert eval_res, '`eval_res` should not be a null dict.'
+
         for name, val in eval_res.items():
             runner.log_buffer.output[name] = val
         runner.log_buffer.ready = True

tests/test_runner/test_eval_hook.py

@@ -126,10 +126,24 @@ def test_eval_hook():
 
     with pytest.raises(KeyError):
         # rule must be in keys of rule_map
-        test_dataset = Model()
+        test_dataset = ExampleDataset()
         data_loader = DataLoader(test_dataset)
         EvalHook(data_loader, save_best='auto', rule='unsupport')
 
+    with pytest.raises(AssertionError):
+        # eval_res returned by `dataset.evaluate()` should not be a null dict
+        class _EvalDataset(ExampleDataset):
+
+            def evaluate(self, results, logger=None):
+                return {}
+
+        test_dataset = _EvalDataset()
+        data_loader = DataLoader(test_dataset)
+        eval_hook = EvalHook(data_loader)
+        runner = _build_epoch_runner()
+        runner.register_hook(eval_hook)
+        runner.run([data_loader], [('train', 1)], 1)
+
     test_dataset = ExampleDataset()
     loader = DataLoader(test_dataset)
     model = Model()
@@ -450,7 +464,7 @@ def test_logger(runner, by_epoch, eval_hook_priority):
 
         path = osp.join(tmpdir, next(scandir(tmpdir, '.json')))
         with open(path) as fr:
-            fr.readline()  # skip first line which is hook_msg
+            fr.readline()  # skip the first line which is `hook_msg`
             train_log = json.loads(fr.readline())
             assert train_log['mode'] == 'train' and 'time' in train_log
             val_log = json.loads(fr.readline())