Background generator (#323)

* BackgroundGenerator
* renaming disk cache.

test/assets/commonvoice/train.tsv

 client_id	path	sentence	up_votes	down_votes	age	gender	accent
 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000	common_voice_tt_00000000.mp3	test.	1	0	thirties	female	
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001	common_voice_tt_00000000.mp3	test.	1	0	thirties	female	

test/test_datasets.py

@@ -3,7 +3,7 @@ import unittest
 
 from torchaudio.datasets.commonvoice import COMMONVOICE
 from torchaudio.datasets.librispeech import LIBRISPEECH
-from torchaudio.datasets.utils import DiskCache
+from torchaudio.datasets.utils import diskcache_iterator, bg_iterator
 from torchaudio.datasets.vctk import VCTK
 from torchaudio.datasets.yesno import YESNO
 
@@ -34,12 +34,19 @@ class TestDatasets(unittest.TestCase):
     def test_commonvoice_diskcache(self):
         path = os.path.join(self.path, "commonvoice")
         data = COMMONVOICE(path, "train.tsv", "tatar")
-        data = DiskCache(data)
+        data = diskcache_iterator(data)
         # Save
         data[0]
         # Load
         data[0]
 
+    def test_commonvoice_bg(self):
+        path = os.path.join(self.path, "commonvoice")
+        data = COMMONVOICE(path, "train.tsv", "tatar")
+        data = bg_iterator(data, 5)
+        for d in data:
+            pass
+
 
 if __name__ == "__main__":
     unittest.main()

torchaudio/datasets/__init__.py

 from .commonvoice import COMMONVOICE
 from .librispeech import LIBRISPEECH
+from .utils import bg_iterator, diskcache_iterator
 from .vctk import VCTK
 from .yesno import YESNO
-from .utils import DiskCache
 
-__all__ = ("COMMONVOICE", "LIBRISPEECH", "VCTK", "YESNO", "DiskCache")
+__all__ = (
+    "COMMONVOICE",
+    "LIBRISPEECH",
+    "VCTK",
+    "YESNO",
+    "diskcache_iterator",
+    "bg_iterator",
+)

torchaudio/datasets/utils.py

@@ -6,7 +6,9 @@ import logging
 import os
 import sys
 import tarfile
+import threading
 import zipfile
+from queue import Queue
 
 import six
 import torch
@@ -192,7 +194,7 @@ def walk_files(root, suffix, prefix=False, remove_suffix=False):
                 yield f
 
 
-class DiskCache(Dataset):
+class _DiskCache(Dataset):
     """
     Wrap a dataset so that, whenever a new item is returned, it is saved to disk.
     """
@@ -221,3 +223,45 @@ class DiskCache(Dataset):
 
     def __len__(self):
         return len(self.dataset)
+
+
+def diskcache_iterator(dataset, location=".cached"):
+    return _DiskCache(dataset, location)
+
+
+class _ThreadedIterator(threading.Thread):
+    """
+    Prefetch the next queue_length items from iterator in a background thread.
+
+    Example:
+    >> for i in bg_iterator(range(10)):
+    >>     print(i)
+    """
+
+    class _End:
+        pass
+
+    def __init__(self, generator, maxsize):
+        threading.Thread.__init__(self)
+        self.queue = Queue(maxsize)
+        self.generator = generator
+        self.daemon = True
+        self.start()
+
+    def run(self):
+        for item in self.generator:
+            self.queue.put(item)
+        self.queue.put(self._End)
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        next_item = self.queue.get()
+        if next_item == self._End:
+            raise StopIteration
+        return next_item
+
+
+def bg_iterator(iterable, maxsize):
+    return _ThreadedIterator(iterable, maxsize=maxsize)