List backends dynamically based on availability

test/common_utils.py

@@ -10,7 +10,7 @@
 import torchaudio
 
 _TEST_DIR_PATH = os.path.dirname(os.path.realpath(__file__))
-BACKENDS = torchaudio._backend._audio_backends
+BACKENDS = torchaudio._backend._BACKENDS
 
 
 def get_asset_path(*paths):

torchaudio/_backend.py

@@ -1,43 +1,49 @@
-from typing import Any
+from typing import Any, Optional
 
-import platform
+from torchaudio._internal import module_utils as _mod_utils
+from . import _soundfile_backend, _sox_backend
 
+_BACKEND = None
+_BACKENDS = {}
 
-from . import _soundfile_backend, _sox_backend
+if _mod_utils.is_module_available('soundfile'):
+    _BACKENDS['soundfile'] = _soundfile_backend
+    _BACKEND = 'soundfile'
+if _mod_utils.is_module_available('torchaudio._torchaudio'):
+    _BACKENDS['sox'] = _sox_backend
+    _BACKEND = 'sox'
 
 
-if platform.system() == "Windows":
-    _audio_backend = "soundfile"
-    _audio_backends = {"soundfile": _soundfile_backend}
-else:
-    _audio_backend = "sox"
-    _audio_backends = {"sox": _sox_backend, "soundfile": _soundfile_backend}
+def list_audio_backends():
+    return list(_BACKENDS.keys())
 
 
 def set_audio_backend(backend: str) -> None:
     """
     Specifies the package used to load.
     Args:
-        backend (str): Name of the backend. One of {}.
-    """.format(_audio_backends.keys())
-    global _audio_backend
-    if backend not in _audio_backends:
-        raise ValueError(
-            "Invalid backend '{}'. Options are {}.".format(backend, _audio_backends.keys())
-        )
-    _audio_backend = backend
+        backend (str): Name of the backend. One of "sox" or "soundfile",
+            based on availability of the system.
+    """
+    if backend not in _BACKENDS:
+        raise RuntimeError(
+            f'Backend "{backend}" is not one of '
+            f'available backends: {list_audio_backends()}.')
+    global _BACKEND
+    _BACKEND = backend
 
 
-def get_audio_backend() -> str:
+def get_audio_backend() -> Optional[str]:
     """
     Gets the name of the package used to load.
     """
-    return _audio_backend
+    return _BACKEND
 
 
 def _get_audio_backend_module() -> Any:
     """
     Gets the module backend to load.
     """
-    backend = get_audio_backend()
-    return _audio_backends[backend]
+    if _BACKEND is None:
+        raise RuntimeError('Backend is not initialized.')
+    return _BACKENDS[_BACKEND]