Fix import error in serialization checks (#241)

* Fix import error

* add test

src/litdata/streaming/serializers.py

@@ -80,10 +80,13 @@ def deserialize(cls, data: bytes) -> Any:
         return Image.frombytes(mode, size, raw)  # pyright: ignore
 
     def can_serialize(self, item: Any) -> bool:
+        if not _PIL_AVAILABLE:
+            return False
+
         from PIL import Image
         from PIL.JpegImagePlugin import JpegImageFile
 
-        return bool(_PIL_AVAILABLE) and isinstance(item, Image.Image) and not isinstance(item, JpegImageFile)
+        return isinstance(item, Image.Image) and not isinstance(item, JpegImageFile)
 
 
 class JPEGSerializer(Serializer):
@@ -137,9 +140,12 @@ def deserialize(self, data: bytes) -> Union["JpegImageFile", torch.Tensor]:
         return img
 
     def can_serialize(self, item: Any) -> bool:
+        if not _PIL_AVAILABLE:
+            return False
+
         from PIL.JpegImagePlugin import JpegImageFile
 
-        return bool(_PIL_AVAILABLE) and isinstance(item, JpegImageFile)
+        return isinstance(item, JpegImageFile)
 
 
 class BytesSerializer(Serializer):

tests/streaming/test_serializer.py

@@ -16,6 +16,7 @@
 import random
 import sys
 from time import time
+from unittest import mock
 
 import numpy as np
 import pytest
@@ -100,6 +101,12 @@ def test_pil_serializer(mode):
     assert np.array_equal(np_data, np_dec_data)
 
 
+def test_pil_serializer_available():
+    serializer = PILSerializer()
+    with mock.patch("litdata.streaming.serializers._PIL_AVAILABLE", False):
+        assert not serializer.can_serialize(None)
+
+
 @pytest.mark.skipif(condition=not _PIL_AVAILABLE, reason="Requires: ['pil']")
 def test_jpeg_serializer():
     serializer = JPEGSerializer()
@@ -121,6 +128,12 @@ def test_jpeg_serializer():
     assert deserialized_img.shape == torch.Size([3, 28, 28])
 
 
+def test_jpeg_serializer_available():
+    serializer = JPEGSerializer()
+    with mock.patch("litdata.streaming.serializers._PIL_AVAILABLE", False):
+        assert not serializer.can_serialize(None)
+
+
 @pytest.mark.flaky(reruns=3)
 @pytest.mark.skipif(sys.platform == "win32", reason="Not supported on windows")
 def test_tensor_serializer():