Update type hints for multiple dataloaders in .fit() and .test() (#1723)

* update typehints

* change log

CHANGELOG.md

@@ -12,6 +12,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Added transfer learning example (for a binary classification task in computer vision) ([#1564](https://github.com/PyTorchLightning/pytorch-lightning/pull/1564))
 
+- Added type hints in `Trainer.fit()` and `Trainer.test()` to reflect that also a list of dataloaders can be passed in ([#1723](https://github.com/PyTorchLightning/pytorch-lightning/pull/1723)).
+
 ### Changed
 
 - Reduction when `batch_size < num_gpus` ([#1609](https://github.com/PyTorchLightning/pytorch-lightning/pull/1609))

pytorch_lightning/trainer/trainer.py

@@ -672,7 +672,7 @@ def fit(
             self,
             model: LightningModule,
             train_dataloader: Optional[DataLoader] = None,
-            val_dataloaders: Optional[DataLoader] = None
+            val_dataloaders: Optional[Union[DataLoader, List[DataLoader]]] = None
     ):
         r"""
         Runs the full optimization routine.
@@ -913,7 +913,11 @@ def run_pretrain_routine(self, model: LightningModule):
         # CORE TRAINING LOOP
         self.train()
 
-    def test(self, model: Optional[LightningModule] = None, test_dataloaders: Optional[DataLoader] = None):
+    def test(
+            self,
+            model: Optional[LightningModule] = None,
+            test_dataloaders: Optional[Union[DataLoader, List[DataLoader]]] = None
+    ):
         r"""
 
         Separates from fit to make sure you never run on your test set until you want to.

tests/trainer/test_dataloaders.py

@@ -113,7 +113,7 @@ class CurrentTestModel(
     trainer.fit(model)
     trainer.test()
 
-    # verify there are 2 val loaders
+    # verify there are 2 test loaders
     assert len(trainer.test_dataloaders) == 2, \
         'Multiple test_dataloaders not initiated properly'
 
@@ -125,7 +125,7 @@ class CurrentTestModel(
     trainer.test()
 
 
-def test_train_dataloaders_passed_to_fit(tmpdir):
+def test_train_dataloader_passed_to_fit(tmpdir):
     """Verify that train dataloader can be passed to fit """
 
     class CurrentTestModel(LightTrainDataloader, TestModelBase):
@@ -175,7 +175,7 @@ class CurrentTestModel(
 
 
 def test_all_dataloaders_passed_to_fit(tmpdir):
-    """Verify train, val & test dataloader can be passed to fit """
+    """Verify train, val & test dataloader(s) can be passed to fit and test method"""
 
     class CurrentTestModel(
         LightTrainDataloader,