Fix val_split

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 prediction would sometimes give the wrong number of outputs ([#1077](https://github.com/PyTorchLightning/lightning-flash/pull/1077))
 
+- Fixed a bug where passing the `val_split` to the `DataModule` would not have the desired effect ([#1079](https://github.com/PyTorchLightning/lightning-flash/pull/1079))
+
 ### Removed
 
 ## [0.6.0] - 2021-13-12

flash/core/data/data_module.py

@@ -110,6 +110,14 @@ def __init__(
         self._test_input = test_input
         self._predict_input = predict_input
 
+        if self._train_input and self._val_input and isinstance(val_split, float) and val_split > 0:
+            raise MisconfigurationException(
+                "A `val_dataset` was provided with `val_split`. Please, choose one or the other."
+            )
+
+        if self._train_input and (val_split is not None and not self._val_input):
+            self._train_input, self._val_input = self._split_train_val(self._train_input, val_split)
+
         self._data_fetcher: Optional[BaseDataFetcher] = data_fetcher or self.configure_data_fetcher()
 
         self._train_dataloader_collate_fn = self._resolve_dataloader_collate_fn(self._train_input)
@@ -122,14 +130,6 @@ def __init__(
         self._test_on_after_batch_transfer_fn = self._resolve_on_after_batch_transfer_fn(self._test_input)
         self._predict_on_after_batch_transfer_fn = self._resolve_on_after_batch_transfer_fn(self._predict_input)
 
-        if self._train_input and self._val_input and isinstance(val_split, float) and val_split > 0:
-            raise MisconfigurationException(
-                "A `val_dataset` was provided with `val_split`. Please, choose one or the other."
-            )
-
-        if self._train_input is not None and (val_split is not None and self._val_input is None):
-            self._train_input, self._val_input = self._split_train_val(self._train_input, val_split)
-
         if self._train_input:
             self.train_dataloader = self._train_dataloader
 

tests/core/data/test_data_module.py

@@ -439,3 +439,15 @@ def test_dataloaders_with_sampler(mock_dataloader):
     for dl in [datamodule.val_dataloader(), datamodule.test_dataloader()]:
         kwargs = mock_dataloader.call_args[1]
         assert "sampler" not in kwargs
+
+
+def test_val_split():
+    datamodule = DataModule(
+        Input(RunningStage.TRAINING, [1] * 100),
+        batch_size=2,
+        num_workers=0,
+        val_split=0.2,
+    )
+
+    assert len(datamodule.train_dataset) == 80
+    assert len(datamodule.val_dataset) == 20