Disable train dataloader shuffle when overfit_batches is active. (#3501)

* Disable train dataloader shuffle when overfit_batches is active.

* pep8

Co-authored-by: William Falcon <waf2107@columbia.edu>

CHANGELOG.md

@@ -51,6 +51,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed getting `experiment_id` from MLFlow only once instead of each training loop ([#3394](https://github.com/PyTorchLightning/pytorch-lightning/pull/3394))
 
+- Fixed overfit_batches which now correctly disables shuffling for the training loader. ([#3501](https://github.com/PyTorchLightning/pytorch-lightning/pull/3501))
+
 ## [0.9.0] - YYYY-MM-DD
 
 ### Added

pytorch_lightning/trainer/data_loading.py

@@ -167,6 +167,12 @@ def reset_train_dataloader(self, model: LightningModule) -> None:
             model: The current `LightningModule`
         """
         self.train_dataloader = self.request_dataloader(model.train_dataloader)
+        if (self.overfit_batches > 0):
+            if hasattr(self.train_dataloader, 'sampler') and isinstance(self.train_dataloader.sampler, RandomSampler):
+                rank_zero_warn('You requested to overfit but enabled training dataloader shuffling.'
+                               ' We are turning it off for you.')
+                self.train_dataloader = self.replace_sampler(
+                    self.train_dataloader, SequentialSampler(self.train_dataloader.dataset))
 
         # debugging
         self.dev_debugger.track_load_dataloader_call('train_dataloader', dataloaders=[self.train_dataloader])
@@ -247,7 +253,7 @@ def _reset_eval_dataloader(
 
                 # when overfitting, the dataloader should not have sampler
                 if self.overfit_batches > 0:
-                    rank_zero_warn('You requested to overfit but enabled training dataloader shuffling.'
+                    rank_zero_warn('You requested to overfit but enabled test/val dataloader shuffling.'
                                    ' We are turning it off for you.')
                     dataloaders[loader_i] = self.replace_sampler(loader, SequentialSampler(loader.dataset))
 

tests/trainer/test_trainer_tricks.py

@@ -56,11 +56,13 @@ def test_overfit_batch_limits(tmpdir):
     # ------------------------------------------------------
     # get the training loader and batch
     # ------------------------------------------------------
+    # Create a reference train dataloader without shuffling.
     train_loader = DataLoader(model.train_dataloader().dataset, shuffle=False)
+    (xa, ya) = next(iter(train_loader))
+    train_loader = DataLoader(model.train_dataloader().dataset, shuffle=True)
     full_train_samples = len(train_loader)
     num_train_samples = int(0.11 * full_train_samples)
 
-    (xa, ya) = next(iter(train_loader))
 
     # ------------------------------------------------------
     # set VAL and Test loaders
@@ -87,7 +89,8 @@ def test_overfit_batch_limits(tmpdir):
 
     trainer = Trainer(overfit_batches=0.11)
     trainer.reset_train_dataloader(model)
-    assert trainer.train_dataloader is train_loader
+    # The dataloader should have been overwritten with a Sequential sampler.
+    assert trainer.train_dataloader is not train_loader
     assert trainer.num_training_batches == num_train_samples
 
     # make sure the loaders are the same