Addresses issue #145 as per @fmessa's suggestion. (#527)

* Addresses issue #145 as per @fmessa's suggestion.

* Removed blank line for styling.

torchvision/datasets/folder.py

@@ -32,17 +32,10 @@ def is_image_file(filename):
     return has_file_allowed_extension(filename, IMG_EXTENSIONS)
 
 
-def find_classes(dir):
-    classes = [d for d in os.listdir(dir) if os.path.isdir(os.path.join(dir, d))]
-    classes.sort()
-    class_to_idx = {classes[i]: i for i in range(len(classes))}
-    return classes, class_to_idx
-
-
 def make_dataset(dir, class_to_idx, extensions):
     images = []
     dir = os.path.expanduser(dir)
-    for target in sorted(os.listdir(dir)):
+    for target in sorted(class_to_idx.keys()):
         d = os.path.join(dir, target)
         if not os.path.isdir(d):
             continue
@@ -86,7 +79,7 @@ class DatasetFolder(data.Dataset):
     """
 
     def __init__(self, root, loader, extensions, transform=None, target_transform=None):
-        classes, class_to_idx = find_classes(root)
+        classes, class_to_idx = self._find_classes(root)
         samples = make_dataset(root, class_to_idx, extensions)
         if len(samples) == 0:
             raise(RuntimeError("Found 0 files in subfolders of: " + root + "\n"
@@ -104,6 +97,24 @@ class DatasetFolder(data.Dataset):
         self.transform = transform
         self.target_transform = target_transform
 
+    def _find_classes(self, dir):
+        """
+        Finds the class folders in a dataset.
+
+        Args:
+            dir (string): Root directory path.
+
+        Returns:
+            tuple: (classes, class_to_idx) where classes are relative to (dir), and class_to_idx is a dictionary.
+
+        Ensures:
+            No class is a subdirectory of another.
+        """
+        classes = [d for d in os.listdir(dir) if os.path.isdir(os.path.join(dir, d))]
+        classes.sort()
+        class_to_idx = {classes[i]: i for i in range(len(classes))}
+        return classes, class_to_idx
+
     def __getitem__(self, index):
         """
         Args: