Fix sampler instance support

CHANGELOG.md

@@ -24,6 +24,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed a bug where buffers in loss functions were not correctly registered in the `Task` ([#1203](https://github.com/PyTorchLightning/lightning-flash/pull/1203))
 
+- Fixed support for passing a sampler instance to `from_*` methods / the `DataModule` ([#1204](https://github.com/PyTorchLightning/lightning-flash/pull/1204))
+
 ## [0.7.0] - 2022-02-15
 
 ### Added

flash/core/data/data_module.py

@@ -69,6 +69,37 @@ class DataModule(pl.LightningDataModule):
         num_workers: The number of workers to use for parallelized loading.
         sampler: A sampler following the :class:`~torch.utils.data.sampler.Sampler` type.
             Will be passed to the DataLoader for the training dataset. Defaults to None.
+
+    Examples
+    ________
+
+    .. testsetup::
+
+        >>> from flash import DataModule
+        >>> from flash.core.utilities.stages import RunningStage
+        >>> from torch.utils.data.sampler import SequentialSampler, WeightedRandomSampler
+        >>> class TestInput(Input):
+        ...     def train_load_data(self, _):
+        ...         return [(0, 1, 2, 3), (0, 1, 2, 3)]
+        >>> train_input = TestInput(RunningStage.TRAINING, [1])
+
+    You can provide the sampler to use for the train dataloader using the ``sampler`` argument.
+    The sampler can be a function or type that needs the dataset as an argument:
+
+    .. doctest::
+
+        >>> datamodule = DataModule(train_input, sampler=SequentialSampler, batch_size=1)
+        >>> print(datamodule.train_dataloader().sampler)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
+        <torch.utils.data.sampler.SequentialSampler object at ...>
+
+    Alternatively, you can pass a sampler instance:
+
+    .. doctest::
+
+        >>> datamodule = DataModule(train_input, sampler=WeightedRandomSampler([0.1, 0.5], 2), batch_size=1)
+        >>> print(datamodule.train_dataloader().sampler)  # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
+        <torch.utils.data.sampler.WeightedRandomSampler object at ...>
+
     """
 
     input_transform_cls = InputTransform
@@ -84,7 +115,7 @@ def __init__(
         val_split: Optional[float] = None,
         batch_size: Optional[int] = None,
         num_workers: int = 0,
-        sampler: Optional[Type[Sampler]] = None,
+        sampler: Optional[Union[Callable, Sampler, Type[Sampler]]] = None,
         pin_memory: bool = True,
         persistent_workers: bool = False,
     ) -> None:
@@ -206,8 +237,10 @@ def _train_dataloader(self) -> DataLoader:
         if self.sampler is None:
             sampler = None
             shuffle = not isinstance(train_ds, IterableDataset)
-        else:
+        elif callable(self.sampler):
             sampler = self.sampler(train_ds)
+        else:
+            sampler = self.sampler
 
         if isinstance(getattr(self, "trainer", None), pl.Trainer):
             dataloader = self.trainer.lightning_module.process_train_dataset(

tests/core/data/test_data_module.py

@@ -419,22 +419,28 @@ def validation_step(self, batch, batch_idx):
     trainer.fit(model, datamodule=datamodule)
 
 
+@pytest.mark.parametrize("sampler, callable", [(mock.MagicMock(), True), (mock.NonCallableMock(), False)])
 @mock.patch("flash.core.data.data_module.DataLoader")
-def test_dataloaders_with_sampler(mock_dataloader):
-    mock_sampler = mock.MagicMock()
+def test_dataloaders_with_sampler(mock_dataloader, sampler, callable):
+    train_input = TestInput(RunningStage.TRAINING, [1])
     datamodule = DataModule(
-        TestInput(RunningStage.TRAINING, [1]),
+        train_input,
         TestInput(RunningStage.VALIDATING, [1]),
         TestInput(RunningStage.TESTING, [1]),
         batch_size=2,
         num_workers=0,
-        sampler=mock_sampler,
+        sampler=sampler,
     )
-    assert datamodule.sampler is mock_sampler
+
+    assert datamodule.sampler is sampler
     dl = datamodule.train_dataloader()
+
+    if callable:
+        sampler.assert_called_once_with(train_input)
+
     kwargs = mock_dataloader.call_args[1]
     assert "sampler" in kwargs
-    assert kwargs["sampler"] is mock_sampler.return_value
+    assert kwargs["sampler"] is (sampler.return_value if callable else sampler)
     for dl in [datamodule.val_dataloader(), datamodule.test_dataloader()]:
         kwargs = mock_dataloader.call_args[1]
         assert "sampler" not in kwargs