Changed GTZAN so that it only traverses filenames belonging to the dataset (#791)

* Addressed review issues in PR #668

* Changed GTZAN so that it only traverses filenames belonging to the dataset

Now, instead of walking the whole directory and subdirectories of the dataset
GTZAN only looks for files under a `genre`/`genre`.`5 digit number`.wav format, where `genre` is an allowed GTZAN genre label.
This allows moving or removing files from the dataset (e.g. for fixing duplication or mislabeling issues).

torchaudio/datasets/gtzan.py

@@ -8,7 +8,6 @@ from torch.utils.data import Dataset
 from torchaudio.datasets.utils import (
     download_url,
     extract_archive,
-    walk_files,
 )
 
 # The following lists prefixed with `filtered_` provide a filtered split
@@ -22,6 +21,19 @@ from torchaudio.datasets.utils import (
 # Those are used when GTZAN is initialised with the `filtered` keyword.
 # The split was taken from (github) jordipons/sklearn-audio-transfer-learning.
 
+gtzan_genres = [
+    "blues",
+    "classical",
+    "country",
+    "disco",
+    "hiphop",
+    "jazz",
+    "metal",
+    "pop",
+    "reggae",
+    "rock",
+]
+
 filtered_test = [
     "blues.00012",
     "blues.00013",
@@ -964,7 +976,9 @@ filtered_valid = [
 
 URL = "http://opihi.cs.uvic.ca/sound/genres.tar.gz"
 FOLDER_IN_ARCHIVE = "genres"
-_CHECKSUMS = {"http://opihi.cs.uvic.ca/sound/genres.tar.gz": "5b3d6dddb579ab49814ab86dba69e7c7"}
+_CHECKSUMS = {
+    "http://opihi.cs.uvic.ca/sound/genres.tar.gz": "5b3d6dddb579ab49814ab86dba69e7c7"
+}
 
 
 def load_gtzan_item(fileid: str, path: str, ext_audio: str) -> Tuple[Tensor, str]:
@@ -1032,10 +1046,32 @@ class GTZAN(Dataset):
             )
 
         if self.subset is None:
-            walker = walk_files(
-                self._path, suffix=self._ext_audio, prefix=False, remove_suffix=True
-            )
-            self._walker = list(walker)
+            # Check every subdirectory under dataset root
+            # which has the same name as the genres in
+            # GTZAN (e.g. `root_dir'/blues/, `root_dir'/rock, etc.)
+            # This lets users remove or move around song files,
+            # useful when e.g. they want to use only some of the files
+            # in a genre or want to label other files with a different
+            # genre.
+            self._walker = []
+
+            root = os.path.expanduser(self._path)
+
+            for directory in gtzan_genres:
+                fulldir = os.path.join(root, directory)
+
+                if not os.path.exists(fulldir):
+                    continue
+
+                songs_in_genre = os.listdir(fulldir)
+                for fname in songs_in_genre:
+                    name, ext = os.path.splitext(fname)
+                    if ext.lower() == ".wav" and "." in name:
+                        # Check whether the file is of the form
+                        # `gtzan_genre`.`5 digit number`.wav
+                        genre, num = name.split(".")
+                        if genre in gtzan_genres and len(num) == 5 and num.isdigit():
+                            self._walker.append(name)
         else:
             if self.subset == "training":
                 self._walker = filtered_train