fix voc difficult not found & map_res empty (#2714)

ppdet/data/source/voc.py

@@ -117,7 +117,12 @@ class VOCDataSet(DetDataset):
                 difficult = []
                 for i, obj in enumerate(objs):
                     cname = obj.find('name').text
-                    _difficult = int(obj.find('difficult').text)
+
+                    # user dataset may not contain difficult field
+                    _difficult = obj.find('difficult')
+                    _difficult = int(
+                        _difficult.text) if _difficult is not None else 0
+
                     x1 = float(obj.find('bndbox').find('xmin').text)
                     y1 = float(obj.find('bndbox').find('ymin').text)
                     x2 = float(obj.find('bndbox').find('xmax').text)

ppdet/engine/callbacks.py

@@ -179,6 +179,11 @@ class Checkpointer(Callback):
                     for metric in self.model._metrics:
                         map_res = metric.get_results()
                         key = 'bbox' if 'bbox' in map_res else 'mask'
+                        if key not in map_res:
+                            logger.warn("Evaluation results empty, this may be due to " \
+                                        "training iterations being too few or not " \
+                                        "loading the correct weights.")
+                            return
                         if map_res[key][0] > self.best_ap:
                             self.best_ap = map_res[key][0]
                             save_name = 'best_model'