Fix DatasetFolder error message (#2143)

* only display extensions if available

* add tests

* fix lint

test/test_datasets.py

@@ -84,6 +84,16 @@ class Tester(unittest.TestCase):
             outputs = sorted([dataset[i] for i in range(len(dataset))])
             self.assertEqual(imgs, outputs)
 
+    def test_imagefolder_empty(self):
+        with get_tmp_dir() as root:
+            with self.assertRaises(RuntimeError):
+                torchvision.datasets.ImageFolder(root, loader=lambda x: x)
+
+            with self.assertRaises(RuntimeError):
+                torchvision.datasets.ImageFolder(
+                    root, loader=lambda x: x, is_valid_file=lambda x: False
+                )
+
     @mock.patch('torchvision.datasets.mnist.download_and_extract_archive')
     def test_mnist(self, mock_download_extract):
         num_examples = 30

torchvision/datasets/folder.py

@@ -94,8 +94,10 @@ class DatasetFolder(VisionDataset):
         classes, class_to_idx = self._find_classes(self.root)
         samples = make_dataset(self.root, class_to_idx, extensions, is_valid_file)
         if len(samples) == 0:
-            raise (RuntimeError("Found 0 files in subfolders of: " + self.root + "\n"
-                                "Supported extensions are: " + ",".join(extensions)))
+            msg = "Found 0 files in subfolders of: {}\n".format(self.root)
+            if extensions is not None:
+                msg += "Supported extensions are: {}".format(",".join(extensions))
+            raise RuntimeError(msg)
 
         self.loader = loader
         self.extensions = extensions