Raise a MisconfigurationException when trainer functions are called with ckpt_path="best" but checkpoint_callback isn't configured

CHANGELOG.md

@@ -172,6 +172,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Enabled automatic parameters tying for TPUs ([#9525](https://github.com/PyTorchLightning/pytorch-lightning/pull/9525))
 
 
+- Added a check to look for `checkpoint_callback` if `ckpt_path="best"` ([#9841](https://github.com/PyTorchLightning/pytorch-lightning/pull/9841))
+
+
 ### Changed
 
 - `pytorch_lightning.loggers.neptune.NeptuneLogger` is now consistent with new [neptune-client](https://github.com/neptune-ai/neptune-client) API ([#6867](https://github.com/PyTorchLightning/pytorch-lightning/pull/6867)).

pytorch_lightning/trainer/trainer.py

@@ -651,7 +651,8 @@ def validate(
 
             ckpt_path: Either ``best`` or path to the checkpoint you wish to validate.
                 If ``None`` and the model instance was passed, use the current weights.
-                Otherwise, the best model from the previous ``trainer.fit`` call will be loaded.
+                Otherwise, the best model from the previous ``trainer.fit`` call will be loaded
+                if a checkpoint callback is configured.
 
             verbose: If True, prints the validation results.
 
@@ -740,7 +741,8 @@ def test(
 
             ckpt_path: Either ``best`` or path to the checkpoint you wish to test.
                 If ``None`` and the model instance was passed, use the current weights.
-                Otherwise, the best model from the previous ``trainer.fit`` call will be loaded.
+                Otherwise, the best model from the previous ``trainer.fit`` call will be loaded
+                if a checkpoint callback is configured.
 
             verbose: If True, prints the test results.
 
@@ -834,7 +836,8 @@ def predict(
 
             ckpt_path: Either ``best`` or path to the checkpoint you wish to predict.
                 If ``None`` and the model instance was passed, use the current weights.
-                Otherwise, the best model from the previous ``trainer.fit`` call will be loaded.
+                Otherwise, the best model from the previous ``trainer.fit`` call will be loaded
+                if a checkpoint callback is configured.
 
         Returns:
             Returns a list of dictionaries, one for each provided dataloader containing their respective predictions.
@@ -1258,15 +1261,20 @@ def __set_ckpt_path(self, ckpt_path: Optional[str], model_provided: bool, model_
 
         if model_connected and ckpt_path is None:
             rank_zero_warn(
-                f"`.{fn}(ckpt_path=None)` was called without a model. "
-                "The best model of the previous `fit` call will be used. "
-                f"You can pass `{fn}(ckpt_path='best')` to avoid this warning "
-                "or `ckpt_path=trainer.model_checkpoint.last_model_path` to use the last model."
+                f"`.{fn}(ckpt_path=None)` was called without a model."
+                " The best model of the previous `fit` call will be used."
+                f" You can pass `{fn}(ckpt_path='best')` to use and best model"
+                " checkpoint and avoid this warning or"
+                " `ckpt_path=trainer.model_checkpoint.last_model_path` to use the last model."
             )
             ckpt_path = "best"
 
         if ckpt_path == "best":
             # if user requests the best checkpoint but we don't have it, error
+            if self.checkpoint_callback is None:
+                raise MisconfigurationException(
+                    f'`.{fn}(ckpt_path="best")` is set but `ModelCheckpoint` is not configured.'
+                )
             if not self.checkpoint_callback.best_model_path:
                 if self.fast_dev_run:
                     raise MisconfigurationException(

tests/trainer/test_trainer.py

@@ -785,6 +785,50 @@ def predict_step(self, batch, *_):
             assert getattr(trainer, path_attr) == ckpt_path
 
 
+@pytest.mark.parametrize("checkpoint_callback", (False, True))
+@pytest.mark.parametrize("fn", ("validate", "test", "predict"))
+def test_tested_checkpoint_path_best(tmpdir, checkpoint_callback, fn):
+    class TestModel(BoringModel):
+        def validation_step(self, batch, batch_idx):
+            self.log("foo", -batch_idx)
+            return super().validation_step(batch, batch_idx)
+
+        def test_step(self, *args):
+            return self.validation_step(*args)
+
+        def predict_step(self, batch, *_):
+            return self(batch)
+
+    model = TestModel()
+    model.test_epoch_end = None
+    trainer = Trainer(
+        max_epochs=2,
+        limit_val_batches=1,
+        limit_test_batches=1,
+        limit_predict_batches=1,
+        enable_progress_bar=False,
+        default_root_dir=tmpdir,
+        checkpoint_callback=checkpoint_callback,
+    )
+    trainer.fit(model)
+
+    trainer_fn = getattr(trainer, fn)
+    path_attr = f"{fn}{'d' if fn == 'validate' else 'ed'}_ckpt_path"
+    assert getattr(trainer, path_attr) is None
+
+    if checkpoint_callback:
+        trainer_fn(ckpt_path="best")
+        assert getattr(trainer, path_attr) == trainer.checkpoint_callback.best_model_path
+
+        trainer_fn(model, ckpt_path="best")
+        assert getattr(trainer, path_attr) == trainer.checkpoint_callback.best_model_path
+    else:
+        with pytest.raises(MisconfigurationException, match="`ModelCheckpoint` is not configured."):
+            trainer_fn(ckpt_path="best")
+        with pytest.raises(MisconfigurationException, match="`ModelCheckpoint` is not configured."):
+            trainer_fn(model, ckpt_path="best")
+
+
 def test_disabled_training(tmpdir):
     """Verify that `limit_train_batches=0` disables the training loop unless `fast_dev_run=True`."""