Disable training with zero num_training_batches when insufficient lim…

…it_train_batches (#5703)

* disable training when zero num_train_batches with limit_train_batches

* refactor train skip condition

* fix formatting issues

* fix formatting issues

* ref: test error msg

* fix tests for data loader calls

* fix train dataloader condition

* update limit_train_batches upper range in test comment

* remove model state check test

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pytorch_lightning/trainer/training_loop.py

@@ -91,13 +91,7 @@ def num_optimizers(self):
         return num_optimizers
 
     def should_skip_training(self):
-        if self.trainer.current_epoch >= self.trainer.max_epochs:
-            return True
-
-        if self.trainer.limit_train_batches == 0:
-            return True
-
-        return False
+        return self.trainer.current_epoch >= self.trainer.max_epochs or self.trainer.num_training_batches == 0
 
     def on_train_start(self):
         # clear cache before training
@@ -203,7 +197,7 @@ def on_train_epoch_start(self, epoch):
         model = self.trainer.get_model()
 
         # reset train dataloader
-        if self.trainer.reload_dataloaders_every_epoch:
+        if epoch != 0 and self.trainer.reload_dataloaders_every_epoch:
             self.trainer.reset_train_dataloader(model)
 
         # set seed for distributed sampler (enables shuffling for each epoch)
@@ -238,7 +232,7 @@ def on_train_batch_end(self, epoch_output, batch_end_outputs, batch, batch_idx,
         self.trainer.logger_connector.on_train_batch_end()
 
     def reset_train_val_dataloaders(self, model):
-        if not self.trainer.reload_dataloaders_every_epoch:
+        if self.trainer.train_dataloader is None or not self.trainer.reload_dataloaders_every_epoch:
             self.trainer.reset_train_dataloader(model)
 
         if self.trainer.val_dataloaders is None and not self.trainer.reload_dataloaders_every_epoch:

tests/trainer/test_trainer.py

@@ -11,20 +11,22 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from argparse import Namespace
-from copy import deepcopy
 import math
 import os
-from pathlib import Path
 import pickle
 import sys
+from argparse import Namespace
+from copy import deepcopy
+from pathlib import Path
 from unittest.mock import ANY, call, patch
 
 import cloudpickle
-from omegaconf import OmegaConf
 import pytest
 import torch
+from omegaconf import OmegaConf
+from torch.utils.data import DataLoader
 
+import tests.base.develop_utils as tutils
 from pytorch_lightning import Callback, LightningModule, Trainer
 from pytorch_lightning.callbacks import EarlyStopping, ModelCheckpoint
 from pytorch_lightning.core.saving import load_hparams_from_tags_csv, load_hparams_from_yaml, save_hparams_to_tags_csv
@@ -35,8 +37,7 @@
 from pytorch_lightning.utilities import NATIVE_AMP_AVAILABLE
 from pytorch_lightning.utilities.cloud_io import load as pl_load
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
-from tests.base import BoringModel, EvalModelTemplate
-import tests.base.develop_utils as tutils
+from tests.base import BoringModel, EvalModelTemplate, RandomDataset
 
 
 @pytest.mark.parametrize("url_ckpt", [True, False])
@@ -1444,3 +1445,58 @@ def test_trainer_profiler_incorrect_arg_type(profiler):
                        match=r"Only None, bool, str and subclasses of `BaseProfiler`"
                              r" are valid values for `Trainer`'s `profiler` parameter. *"):
         Trainer(profiler=profiler)
+
+
+@pytest.mark.parametrize(
+    ["limit_train_batches", "global_step", "num_training_batches", "current_epoch", "should_train"],
+    [(0.2, 0, 0, 0, False), (0.5, 10, 2, 4, True)],
+)
+def test_disabled_training_for_insufficient_limit_train_batches(tmpdir, limit_train_batches, global_step,
+                                                                num_training_batches, current_epoch, should_train):
+    """
+    Verify when `limit_train_batches` is float & between [0.0, 1.0] and
+    `int(self.num_training_batches * self.limit_train_batches) == 0`, the training loop is disabled.
+    """
+    class CurrentModel(BoringModel):
+
+        training_step_invoked = False
+        training_epoch_end_invoked = False
+
+        def training_step(self, *args, **kwargs):
+            self.training_step_invoked = True
+            return super().training_step(*args, **kwargs)
+
+        def training_epoch_end(self, *args, **kwargs):
+            self.training_epoch_end_invoked = True
+            return super().training_epoch_end(*args, **kwargs)
+
+    dataset_len = 100
+    batch_size = 25
+
+    train = RandomDataset(32, length=dataset_len)
+    train_loader = DataLoader(train, batch_size=batch_size)
+
+    model = CurrentModel()
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=5,
+        limit_train_batches=limit_train_batches,
+    )
+    result = trainer.fit(model, train_loader)
+
+    params_string = f"""`limit_train_batches={limit_train_batches}`, `dataset_len={dataset_len}`
+                        & `batch_size={batch_size}` as
+                        `num_training_batches={num_training_batches}`"""
+    if should_train:
+        error_string = f"should run with {params_string}"
+    else:
+        error_string = f"should not run with {params_string}"
+
+    assert result == 1, "training failed to complete"
+    assert trainer.state == TrainerState.FINISHED
+    assert trainer.global_step == global_step
+    assert trainer.num_training_batches == num_training_batches
+    assert trainer.current_epoch == current_epoch
+    assert model.training_step_invoked == should_train, f"`training_step` {error_string}"
+    assert model.training_epoch_end_invoked == should_train, f"`training_epoch_end` {error_string}"