not assert task.load_data_frame(required_columns={}) (#180)

* not assert task.load_data_frame(required_columns={})

* change dict check method

* add ut

* change ut name

gokart/task.py

@@ -233,7 +233,11 @@ def _flatten_recursively(dfs):
             else:
                 return dfs
 
-        data = _flatten_recursively(self.load(target=target))
+        dfs = self.load(target=target)
+        if isinstance(dfs, dict) and len(dfs) == 1:
+            dfs = list(dfs.values())[0]
+
+        data = _flatten_recursively(dfs)
 
         required_columns = required_columns or set()
         if data.empty and len(data.index) == 0 and len(required_columns - set(data.columns)) > 0:

test/test_task_on_kart.py

@@ -314,7 +314,7 @@ def test_add_cofigureation_evaluation_order(self, mock_cmdline: MagicMock):
         class DummyTaskAddConfiguration(gokart.TaskOnKart):
             aa = luigi.IntParameter()
 
-        luigi.configuration.get_config().set(f'DummyTaskAddConfiguration', 'aa', '3')
+        luigi.configuration.get_config().set('DummyTaskAddConfiguration', 'aa', '3')
         mock_cmdline.return_value = luigi.cmdline_parser.CmdlineParser(['DummyTaskAddConfiguration'])
         self.assertEqual(DummyTaskAddConfiguration().aa, 3)
 
@@ -329,6 +329,14 @@ def test_load_list_of_list_pandas(self):
         self.assertIsInstance(df, pd.DataFrame)
         self.assertEqual(3, df.shape[0])
 
+    def test_load_single_value_dict_of_dataframe(self):
+        task = _DummyTask()
+        task.load = MagicMock(return_value={'a': pd.DataFrame(dict(a=[1]))})
+
+        df = task.load_data_frame()
+        self.assertIsInstance(df, pd.DataFrame)
+        self.assertEqual(1, df.shape[0])
+
     def test_load_data_frame_drop_columns(self):
         task = _DummyTask()
         task.load = MagicMock(return_value=pd.DataFrame(dict(a=[1], b=[2], c=[3])))