Fix AttributeError when using YAMLStoryWriter.stories_to_yaml

rasa/shared/core/training_data/story_writer/yaml_story_writer.py

@@ -89,22 +89,24 @@ def dump(
             is_test_story: Identifies if the stories should be exported in test stories
                            format.
         """
-        self._is_test_story = is_test_story
-
-        result = self.stories_to_yaml(story_steps)
+        result = self.stories_to_yaml(story_steps, is_test_story)
         if is_appendable and KEY_STORIES in result:
             result = result[KEY_STORIES]
 
         rasa.shared.utils.io.write_yaml(result, target, True)
 
-    def stories_to_yaml(self, story_steps: List[StoryStep]) -> Dict[Text, Any]:
+    def stories_to_yaml(
+        self, story_steps: List[StoryStep], is_test_story: bool = False
+    ) -> Dict[Text, Any]:
         """Converts a sequence of story steps into yaml format.
 
         Args:
             story_steps: Original story steps to be converted to the YAML.
         """
         from rasa.shared.utils.validation import KEY_TRAINING_DATA_FORMAT_VERSION
 
+        self._is_test_story = is_test_story
+
         stories = []
         rules = []
         for story_step in story_steps:

tests/shared/core/training_data/story_writer/test_yaml_story_writer.py

@@ -167,3 +167,18 @@ async def test_action_start_action_listen_are_not_dumped():
 
     assert ACTION_SESSION_START_NAME not in dump
     assert ACTION_LISTEN_NAME not in dump
+
+
+def test_yaml_writer_stories_to_yaml(default_domain: Domain):
+    from collections import OrderedDict
+
+    reader = YAMLStoryReader(default_domain, None, False)
+    writer = YAMLStoryWriter()
+    steps = reader.read_from_file(
+        "data/test_yaml_stories/simple_story_with_only_end.yml"
+    )
+
+    result = writer.stories_to_yaml(steps)
+    assert isinstance(result, OrderedDict)
+    assert "stories" in result
+    assert len(result["stories"]) == 1