Allow DataLoaderAdapter subclasses to be pickled by implementing __reduce__

src/accelerate/data_loader.py

@@ -416,25 +416,6 @@ def __init__(self, dataset, use_stateful_dataloader=False, batch_sampler=None, *
         else:
             self.base_dataloader = DataLoader(dataset, batch_sampler=batch_sampler, **kwargs)
 
-        # Dynamically mixin the parent class. See https://stackoverflow.com/a/31075641
-        # In C++ terms, this is analogous to creating `DataLoaderAdapter<T> : T`, where T is a DataLoader or
-        # StatefulDataLoader
-        #
-        # The same functionality could be achieved by directly creating the required subclasses for both {DataLoader,
-        # StatefulDataLoader}, however that could lead to much messier code, with duplicated classes and conditional
-        # dispatching scattered throughout various functions and files.
-        #
-        # This code is incredibly awkward but it's the only way to make `isinstance(obj, StatefulDataLoader)` work
-        # transparently.
-        #
-        # A more robust solution is for DataLoaderAdapter to not inherit from DataLoader (compose rather than inherit),
-        # but this would not be backwards compatible with existing code which assumes
-        # DataLoaderShard/DataLoaderDispatcher are DataLoaders.
-        base_cls = self.__class__
-        base_cls_name = self.__class__.__name__
-        parent_cls_name = self.base_dataloader.__class__
-        self.__class__ = type(base_cls_name, (base_cls, parent_cls_name), {})
-
         if hasattr(self.base_dataloader, "state_dict"):
             self.dl_state_dict = self.base_dataloader.state_dict()
 
@@ -451,6 +432,18 @@ def state_dict(self):
     def load_state_dict(self, state_dict):
         self.base_dataloader.load_state_dict(state_dict)
 
+    @property
+    def __class__(self):
+        """
+        In order to maintain backwards compatability with other code, we need to ensure `isinstance(obj, DataLoader)`
+        returs true. This is because some downstream code assumes that the `DataLoader` is the base class of the
+        object.
+        """
+        return self.base_dataloader.__class__
+
+    def __len__(self):
+        return len(self.base_dataloader)
+
     def adjust_state_dict_for_prefetch(self):
         """
         Adjusts the state dict for prefetching. Natively, this will adjust all of the iters yielded keys in
@@ -580,6 +573,15 @@ def __iter__(self):
         self.iteration += 1
         self.end()
 
+    def __reduce__(self):
+        """
+        Define the `__reduce__` method to ensure a `DataLoaderShard` can be pickled and unpickled. This needs to be
+        explicitly defined since default pickling behavior is broken by `DataLoaderAdapter` messing with its
+        `__class__` member.
+        """
+        args = super().__reduce__()
+        return (DataLoaderShard, *args[1:])
+
     def set_epoch(self, epoch: int):
         # In case it is manually passed in, the user can set it to what they like
         if self.iteration != epoch:
@@ -865,14 +867,23 @@ def set_epoch(self, epoch: int):
             self.dataset.set_epoch(epoch)
 
     def __len__(self):
-        whole_length = super().__len__()
+        whole_length = len(self.base_dataloader)
         if self.split_batches:
             return whole_length
         elif self._drop_last:
             return whole_length // self.state.num_processes
         else:
             return math.ceil(whole_length / self.state.num_processes)
 
+    def __reduce__(self):
+        """
+        Define the `__reduce__` method to ensure a `DataLoaderDispatcher` can be pickled and unpickled. This needs to
+        be explicitly defined since default pickling behavior is broken by `DataLoaderAdapter` messing with its
+        `__class__` member.
+        """
+        args = super().__reduce__()
+        return (DataLoaderDispatcher, *args[1:])
+
     @property
     def total_batch_size(self):
         return (
@@ -1211,6 +1222,18 @@ def __iter__(self):
                 yield batch
         self.end()
 
+    def __len__(self):
+        return len(self.base_dataloader) - self.skip_batches
+
+    def __reduce__(self):
+        """
+        Define the `__reduce__` method to ensure a `SkipDataLoader` can be pickled and unpickled. This needs to be
+        explicitly defined since default pickling behavior is broken by `DataLoaderAdapter` messing with its
+        `__class__` member.
+        """
+        args = super().__reduce__()
+        return (SkipDataLoader, *args[1:])
+
 
 def skip_first_batches(dataloader, num_batches=0):
     """

src/accelerate/test_utils/scripts/test_distributed_data_loop.py

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import pickle
 import tempfile
 import warnings
 from typing import List
@@ -247,6 +248,16 @@ def test_join_raises_warning_for_iterable_when_overriding_even_batches():
         assert "only supported for map-style datasets" in str(w[-1].message)
 
 
+def test_pickle_accelerator():
+    accelerator = create_accelerator()
+    data_loader = create_dataloader(accelerator, dataset_size=32, batch_size=4)
+    _ = accelerator.prepare(data_loader)
+    pickled_accelerator = pickle.dumps(accelerator)
+    unpickled_accelerator = pickle.loads(pickled_accelerator)
+    # TODO: Maybe this should be implemented as __eq__ for AcceleratorState?
+    assert accelerator.state.__dict__ == unpickled_accelerator.state.__dict__
+
+
 def test_data_loader(data_loader, accelerator):
     # Prepare the DataLoader
     data_loader = accelerator.prepare(data_loader)
@@ -368,6 +379,9 @@ def main():
     test_join_raises_warning_for_non_ddp_distributed(accelerator)
     accelerator.state.distributed_type = original_state
 
+    accelerator.print("Test pickling an accelerator")
+    test_pickle_accelerator()
+
     dataset = DummyDataset()
     # Conventional Dataloader with shuffle=False
     loader = DataLoader(dataset, shuffle=False, batch_size=BATCH_SIZE, num_workers=NUM_WORKERS)

tests/test_accelerator.py

@@ -27,6 +27,7 @@
 
 from accelerate import DistributedType, infer_auto_device_map, init_empty_weights, load_checkpoint_and_dispatch
 from accelerate.accelerator import Accelerator
+from accelerate.data_loader import DataLoaderDispatcher, DataLoaderShard, skip_first_batches
 from accelerate.state import GradientState, PartialState
 from accelerate.test_utils import (
     require_bnb,
@@ -647,6 +648,52 @@ def test_can_unwrap_model(self):
         model_loaded = pickle.loads(pickle.dumps(model))
         model_loaded(inputs)
 
+    @parameterized.expand([True, False])
+    def test_can_pickle_dataloader(self, dispatch_batches):
+        """
+        Test that pickling a prepared dataloader works.
+        """
+        data = torch.arange(10).to(torch_device)
+        ds = torch.utils.data.TensorDataset(data)
+        dl = torch.utils.data.DataLoader(ds)
+        skip_dl = skip_first_batches(dl, 2)
+
+        # Currently, StatefulDataLoader doesn't seem to support pickling, so we aren't testing that functionality
+        # TODO: Add support for pickling StatefulDataLoader
+        dataloader_config = DataLoaderConfiguration(dispatch_batches=dispatch_batches, use_stateful_dataloader=False)
+        accelerator = Accelerator(dataloader_config=dataloader_config)
+
+        original_dl, _ = accelerator.prepare(dl, skip_dl)
+        if dispatch_batches:
+            assert isinstance(original_dl, DataLoaderDispatcher)
+        else:
+            assert isinstance(original_dl, DataLoaderShard)
+
+        prepared_model_dumps = pickle.dumps(accelerator)
+
+        model_loaded = pickle.loads(prepared_model_dumps)
+        assert len(model_loaded._dataloaders) == 2
+
+        # Assert equality of recovered and original dataloader
+        loaded_dl = model_loaded._dataloaders[0]
+        assert isinstance(loaded_dl, DataLoader)
+        if dispatch_batches:
+            assert isinstance(loaded_dl, DataLoaderDispatcher)
+        else:
+            assert isinstance(loaded_dl, DataLoaderShard)
+        assert len(loaded_dl) == len(original_dl)
+        assert [i for i in loaded_dl] == [i for i in original_dl]
+
+        # Test skip dataloader works as expected as well
+        loaded_skip_dl = model_loaded._dataloaders[1]
+        assert isinstance(loaded_skip_dl, DataLoader)
+        if dispatch_batches:
+            assert isinstance(loaded_dl, DataLoaderDispatcher)
+        else:
+            assert isinstance(loaded_dl, DataLoaderShard)
+        assert len(loaded_skip_dl) == len(original_dl) - 2
+        assert [i for i in loaded_skip_dl] == [i for i in original_dl][2:]
+
     # Ideally would be a parameterized test which works with either stateful or non-stateful dataloaders, but dependencies are a bit awkward.
     @require_torchdata_stateful_dataloader
     def test_prepared_objects_are_referenced_with_stateful_dataloader(self):

tests/test_data_loader.py

@@ -420,6 +420,14 @@ def test_dataloader_inheritance(self):
         skip_dl = SkipDataLoader(range(16), batch_size=4, skip_batches=2)
         dl_shard = DataLoaderShard(range(16), batch_size=4)
         dl_dispatcher = DataLoaderDispatcher(range(16), batch_size=4)
+
+        # Test dataloaders are instances of instantiated classes
+        # These asserts look redundant, but it's worth checking since we are doing magic tricks such as dynamically overriding __class__
+        assert isinstance(skip_dl, SkipDataLoader)
+        assert isinstance(dl_shard, DataLoaderShard)
+        assert isinstance(dl_dispatcher, DataLoaderDispatcher)
+
+        # Test dataloaders are instances of base classes
         assert isinstance(skip_dl, DataLoader)
         assert isinstance(dl_shard, DataLoader)
         assert isinstance(dl_dispatcher, DataLoader)
@@ -556,6 +564,13 @@ def test_dataloader_inheritance(self):
         skip_dl = SkipDataLoader(range(16), batch_size=4, skip_batches=2, use_stateful_dataloader=True)
         dl_shard = DataLoaderShard(range(16), batch_size=4, use_stateful_dataloader=True)
         dl_dispatcher = DataLoaderDispatcher(range(16), batch_size=4, use_stateful_dataloader=True)
+
+        # Test dataloaders are instances of instantiated classes
+        # These asserts look redundant, but it's worth checking since we are doing magic tricks such as dynamically overriding __class__
+        assert isinstance(skip_dl, SkipDataLoader)
+        assert isinstance(dl_shard, DataLoaderShard)
+        assert isinstance(dl_dispatcher, DataLoaderDispatcher)
+
         assert isinstance(skip_dl, StatefulDataLoader)
         assert isinstance(dl_shard, StatefulDataLoader)
         assert isinstance(dl_dispatcher, StatefulDataLoader)