fix dataset-flowers random fail. test=develop (#17836)

fix flower-dataset unclose error

python/paddle/dataset/flowers.py

@@ -117,7 +117,8 @@ def reader_creator(data_file,
 
     def reader():
         while True:
-            for file in open(file_list):
+            with open(file_list, 'r') as f_list:
+                for file in f_list:
                     file = file.strip()
                     batch = None
                     with open(file, 'rb') as f:
@@ -125,11 +126,13 @@ def reader_creator(data_file,
                             batch = pickle.load(f)
                         else:
                             batch = pickle.load(f, encoding='bytes')
+
                         if six.PY3:
                             batch = cpt.to_text(batch)
-                data = batch['data']
-                labels = batch['label']
-                for sample, label in six.moves.zip(data, batch['label']):
+                        data_batch = batch['data']
+                        labels_batch = batch['label']
+                        for sample, label in six.moves.zip(data_batch,
+                                                           labels_batch):
                             yield sample, int(label) - 1
             if not cycle:
                 break