Fix incorrect extension parsing in sox_io_backend.save(#885)

* Fix incorrect extension parsing in sox_io_backend.save
* Add tests for compression=None

test/torchaudio_unittest/sox_io_backend/save_test.py

@@ -235,7 +235,7 @@ class TestSave(SaveTestBase):
     @parameterized.expand(list(itertools.product(
         [8000, 16000],
         [1, 2],
-        [-4.2, -0.2, 0, 0.2, 96, 128, 160, 192, 224, 256, 320],
+        [None, -4.2, -0.2, 0, 0.2, 96, 128, 160, 192, 224, 256, 320],
     )), name_func=name_func)
     def test_mp3(self, sample_rate, num_channels, bit_rate):
         """`sox_io_backend.save` can save mp3 format."""
@@ -254,7 +254,7 @@ class TestSave(SaveTestBase):
     @parameterized.expand(list(itertools.product(
         [8000, 16000],
         [1, 2],
-        list(range(9)),
+        [None] + list(range(9)),
     )), name_func=name_func)
     def test_flac(self, sample_rate, num_channels, compression_level):
         """`sox_io_backend.save` can save flac format."""
@@ -273,7 +273,7 @@ class TestSave(SaveTestBase):
     @parameterized.expand(list(itertools.product(
         [8000, 16000],
         [1, 2],
-        [-1, 0, 1, 2, 3, 3.6, 5, 10],
+        [None, -1, 0, 1, 2, 3, 3.6, 5, 10],
     )), name_func=name_func)
     def test_vorbis(self, sample_rate, num_channels, quality_level):
         """`sox_io_backend.save` can save vorbis format."""

torchaudio/backend/sox_io_backend.py

@@ -159,7 +159,7 @@ def save(
             See the detail at http://sox.sourceforge.net/soxformat.html.
     """
     if compression is None:
-        ext = str(filepath)[-3:].lower()
+        ext = str(filepath).split('.')[-1].lower()
         if ext in ['wav', 'sph']:
             compression = 0.
         elif ext == 'mp3':