Fix StreamingDataset.get_len(num_workers=0)

src/litdata/streaming/dataset.py

@@ -197,13 +197,13 @@ def set_num_workers(self, num_workers: int) -> None:
         self.num_workers = num_workers or 1
 
     def get_len(self, num_workers: int, batch_size: int) -> int:
-        self.num_workers = num_workers
-        self.batch_size = batch_size
+        self.set_num_workers(num_workers)
+        self.set_batch_size(batch_size)
         worker_env = _WorkerEnv.detect()
         if self.shuffler is None:
             cache = self._create_cache(worker_env=worker_env)
             self.shuffler = self._create_shuffler(cache)
-        return self.shuffler.get_len(self.distributed_env, num_workers, batch_size, self.current_epoch)
+        return self.shuffler.get_len(self.distributed_env, self.num_workers, self.batch_size, self.current_epoch)
 
     def __iter__(self) -> "StreamingDataset":
         # When the StreamingDataset is used within map or optimize, let's refetch the distributed env.

tests/processing/test_functions.py

@@ -176,7 +176,7 @@ def test_optimize_append_overwrite(tmpdir):
     assert ds[:] == [(i, i**2, i**3) for i in range(0, 5)]
 
 
-@pytest.mark.skipif(sys.platform == "win32" or sys.platform == "darwin", reason="too slow")
+@pytest.mark.skipif(sys.platform == "win32", reason="too slow")
 def test_optimize_checkpoint_in_none_and_append_mode(tmpdir):
     output_dir = str(tmpdir / "output_dir")
 
@@ -188,6 +188,7 @@ def test_optimize_checkpoint_in_none_and_append_mode(tmpdir):
             chunk_size=1,
             num_workers=2,
             use_checkpoint=True,
+            start_method="fork",
         )
 
     # check that the checkpoints are created
@@ -201,6 +202,7 @@ def test_optimize_checkpoint_in_none_and_append_mode(tmpdir):
         chunk_size=1,
         num_workers=2,
         use_checkpoint=True,
+        start_method="fork",
     )
 
     ds = StreamingDataset(output_dir)
@@ -221,6 +223,7 @@ def test_optimize_checkpoint_in_none_and_append_mode(tmpdir):
             num_workers=2,
             use_checkpoint=True,
             mode="append",
+            start_method="fork",
         )
 
     # check that the checkpoints are created
@@ -240,6 +243,7 @@ def test_optimize_checkpoint_in_none_and_append_mode(tmpdir):
         num_workers=2,
         use_checkpoint=True,
         mode="append",
+        start_method="fork",
     )
 
     ds = StreamingDataset(output_dir)

tests/streaming/test_dataloader.py

@@ -3,7 +3,7 @@
 import pytest
 import torch
 from litdata.constants import _VIZ_TRACKER_AVAILABLE
-from litdata.streaming import CombinedStreamingDataset, StreamingDataLoader
+from litdata.streaming import Cache, CombinedStreamingDataset, StreamingDataLoader, StreamingDataset
 from litdata.streaming import dataloader as streaming_dataloader_module
 from torch import tensor
 
@@ -187,3 +187,18 @@ def test_custom_collate_multiworker():
 
     # Try calling the state_dict. No error should follow
     _state_dict = dataloader.state_dict()
+
+
+def test_dataloader_no_workers(tmpdir):
+    cache = Cache(input_dir=str(tmpdir), chunk_bytes="64MB")
+    for i in range(1000):
+        cache[i] = i
+
+    cache.done()
+    cache.merge()
+
+    dataset = StreamingDataset(str(tmpdir), shuffle=True)
+    dataloader = StreamingDataLoader(dataset)
+    assert len(dataset) == 1000
+    assert len(dataloader) == 1000
+    assert len(dataset) == 1000