Refactor codebase to use trainer.loggers over trainer.logger when needed

pl_examples/domain_templates/generative_adversarial_net.py

@@ -206,7 +206,8 @@ def on_train_epoch_end(self):
         # log sampled images
         sample_imgs = self(z)
         grid = torchvision.utils.make_grid(sample_imgs)
-        self.logger.experiment.add_image("generated_images", grid, self.current_epoch)
+        for logger in self.loggers:
+            logger.experiment.add_image("generated_images", grid, self.current_epoch)
 
 
 def main(args: Namespace) -> None:

pytorch_lightning/callbacks/device_stats_monitor.py

@@ -44,7 +44,7 @@ class DeviceStatsMonitor(Callback):
     """
 
     def setup(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule", stage: Optional[str] = None) -> None:
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException("Cannot use DeviceStatsMonitor callback with Trainer that has no logger.")
 
     def on_train_batch_start(
@@ -55,17 +55,18 @@ def on_train_batch_start(
         batch_idx: int,
         unused: Optional[int] = 0,
     ) -> None:
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException("Cannot use `DeviceStatsMonitor` callback with `Trainer(logger=False)`.")
 
         if not trainer.logger_connector.should_update_logs:
             return
 
         device = trainer.strategy.root_device
         device_stats = trainer.accelerator.get_device_stats(device)
-        separator = trainer.logger.group_separator
-        prefixed_device_stats = _prefix_metric_keys(device_stats, "on_train_batch_start", separator)
-        trainer.logger.log_metrics(prefixed_device_stats, step=trainer.global_step)
+        for logger in trainer.loggers:
+            separator = logger.group_separator
+            prefixed_device_stats = _prefix_metric_keys(device_stats, "on_train_batch_start", separator)
+            logger.log_metrics(prefixed_device_stats, step=trainer.global_step)
 
     def on_train_batch_end(
         self,
@@ -76,17 +77,18 @@ def on_train_batch_end(
         batch_idx: int,
         unused: Optional[int] = 0,
     ) -> None:
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException("Cannot use `DeviceStatsMonitor` callback with `Trainer(logger=False)`.")
 
         if not trainer.logger_connector.should_update_logs:
             return
 
         device = trainer.strategy.root_device
         device_stats = trainer.accelerator.get_device_stats(device)
-        separator = trainer.logger.group_separator
-        prefixed_device_stats = _prefix_metric_keys(device_stats, "on_train_batch_end", separator)
-        trainer.logger.log_metrics(prefixed_device_stats, step=trainer.global_step)
+        for logger in trainer.loggers:
+            separator = logger.group_separator
+            prefixed_device_stats = _prefix_metric_keys(device_stats, "on_train_batch_end", separator)
+            logger.log_metrics(prefixed_device_stats, step=trainer.global_step)
 
 
 def _prefix_metric_keys(metrics_dict: Dict[str, float], prefix: str, separator: str) -> Dict[str, float]:

pytorch_lightning/callbacks/gpu_stats_monitor.py

@@ -123,7 +123,7 @@ def __init__(
         self._gpu_ids: List[str] = []  # will be assigned later in setup()
 
     def setup(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule", stage: Optional[str] = None) -> None:
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException("Cannot use GPUStatsMonitor callback with Trainer that has no logger.")
 
         if trainer.strategy.root_device.type != "cuda":
@@ -161,8 +161,8 @@ def on_train_batch_start(
             # First log at beginning of second step
             logs["batch_time/inter_step (ms)"] = (time.time() - self._snap_inter_step_time) * 1000
 
-        assert trainer.logger is not None
-        trainer.logger.log_metrics(logs, step=trainer.global_step)
+        for logger in trainer.loggers:
+            logger.log_metrics(logs, step=trainer.global_step)
 
     @rank_zero_only
     def on_train_batch_end(
@@ -186,8 +186,8 @@ def on_train_batch_end(
         if self._log_stats.intra_step_time and self._snap_intra_step_time:
             logs["batch_time/intra_step (ms)"] = (time.time() - self._snap_intra_step_time) * 1000
 
-        assert trainer.logger is not None
-        trainer.logger.log_metrics(logs, step=trainer.global_step)
+        for logger in trainer.loggers:
+            logger.log_metrics(logs, step=trainer.global_step)
 
     @staticmethod
     def _get_gpu_ids(device_ids: List[int]) -> List[str]:

pytorch_lightning/callbacks/lr_monitor.py

@@ -104,7 +104,7 @@ def on_train_start(self, trainer: "pl.Trainer", *args: Any, **kwargs: Any) -> No
             MisconfigurationException:
                 If ``Trainer`` has no ``logger``.
         """
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException(
                 "Cannot use `LearningRateMonitor` callback with `Trainer` that has no logger."
             )
@@ -149,7 +149,6 @@ def _check_no_key(key: str) -> bool:
         self.last_momentum_values = {name + "-momentum": None for name in names_flatten}
 
     def on_train_batch_start(self, trainer: "pl.Trainer", *args: Any, **kwargs: Any) -> None:
-        assert trainer.logger is not None
         if not trainer.logger_connector.should_update_logs:
             return
 
@@ -158,16 +157,17 @@ def on_train_batch_start(self, trainer: "pl.Trainer", *args: Any, **kwargs: Any)
             latest_stat = self._extract_stats(trainer, interval)
 
             if latest_stat:
-                trainer.logger.log_metrics(latest_stat, step=trainer.global_step)
+                for logger in trainer.loggers:
+                    logger.log_metrics(latest_stat, step=trainer.global_step)
 
     def on_train_epoch_start(self, trainer: "pl.Trainer", *args: Any, **kwargs: Any) -> None:
-        assert trainer.logger is not None
         if self.logging_interval != "step":
             interval = "epoch" if self.logging_interval is None else "any"
             latest_stat = self._extract_stats(trainer, interval)
 
             if latest_stat:
-                trainer.logger.log_metrics(latest_stat, step=trainer.global_step)
+                for logger in trainer.loggers:
+                    logger.log_metrics(latest_stat, step=trainer.global_step)
 
     def _extract_stats(self, trainer: "pl.Trainer", interval: str) -> Dict[str, float]:
         latest_stat = {}

pytorch_lightning/callbacks/model_checkpoint.py

@@ -35,6 +35,7 @@
 from pytorch_lightning.callbacks.base import Callback
 from pytorch_lightning.utilities.cloud_io import get_filesystem
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
+from pytorch_lightning.utilities.logger import _name, _version
 from pytorch_lightning.utilities.rank_zero import rank_zero_info, rank_zero_warn
 from pytorch_lightning.utilities.types import _METRIC, _PATH, STEP_OUTPUT
 from pytorch_lightning.utilities.warnings import WarningCache
@@ -379,8 +380,9 @@ def save_checkpoint(self, trainer: "pl.Trainer") -> None:
         self._save_last_checkpoint(trainer, monitor_candidates)
 
         # notify loggers
-        if trainer.is_global_zero and trainer.logger:
-            trainer.logger.after_save_checkpoint(proxy(self))
+        if trainer.is_global_zero:
+            for logger in trainer.loggers:
+                logger.after_save_checkpoint(proxy(self))
 
     def _should_skip_saving_checkpoint(self, trainer: "pl.Trainer") -> bool:
         from pytorch_lightning.trainer.states import TrainerFn
@@ -572,20 +574,20 @@ def __resolve_ckpt_dir(self, trainer: "pl.Trainer") -> None:
         """
         if self.dirpath is not None:
             return  # short circuit
-
-        if trainer.logger is not None:
+        if trainer.loggers:
             if trainer.weights_save_path != trainer.default_root_dir:
                 # the user has changed weights_save_path, it overrides anything
                 save_dir = trainer.weights_save_path
-            else:
+            elif len(trainer.loggers) == 1:
                 save_dir = trainer.logger.save_dir or trainer.default_root_dir
+            else:
+                save_dir = trainer.default_root_dir
 
-            version = (
-                trainer.logger.version
-                if isinstance(trainer.logger.version, str)
-                else f"version_{trainer.logger.version}"
-            )
-            ckpt_path = os.path.join(save_dir, str(trainer.logger.name), version, "checkpoints")
+            name = _name(trainer.loggers)
+            version = _version(trainer.loggers)
+            version = version if isinstance(version, str) else f"version_{version}"
+
+            ckpt_path = os.path.join(save_dir, str(name), version, "checkpoints")
         else:
             ckpt_path = os.path.join(trainer.weights_save_path, "checkpoints")
 

pytorch_lightning/callbacks/progress/base.py

@@ -15,6 +15,7 @@
 
 import pytorch_lightning as pl
 from pytorch_lightning.callbacks import Callback
+from pytorch_lightning.utilities.logger import _version
 from pytorch_lightning.utilities.rank_zero import rank_zero_warn
 
 
@@ -213,11 +214,12 @@ def get_standard_metrics(trainer: "pl.Trainer", pl_module: "pl.LightningModule")
     if pl_module.truncated_bptt_steps > 0:
         items_dict["split_idx"] = trainer.fit_loop.split_idx
 
-    if trainer.logger is not None and trainer.logger.version is not None:
-        version = trainer.logger.version
-        if isinstance(version, str):
-            # show last 4 places of long version strings
-            version = version[-4:]
-        items_dict["v_num"] = version
+    if trainer.loggers:
+        version = _version(trainer.loggers)
+        if version is not None:
+            if isinstance(version, str):
+                # show last 4 places of long version strings
+                version = version[-4:]
+            items_dict["v_num"] = version
 
     return items_dict

pytorch_lightning/callbacks/xla_stats_monitor.py

@@ -70,7 +70,7 @@ def __init__(self, verbose: bool = True) -> None:
         self._verbose = verbose
 
     def on_train_start(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule") -> None:
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException("Cannot use XLAStatsMonitor callback with Trainer that has no logger.")
 
         if isinstance(trainer.accelerator, TPUAccelerator):
@@ -88,7 +88,7 @@ def on_train_epoch_start(self, trainer: "pl.Trainer", pl_module: "pl.LightningMo
         self._start_time = time.time()
 
     def on_train_epoch_end(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule") -> None:
-        if not trainer.logger:
+        if not trainer.loggers:
             raise MisconfigurationException("Cannot use XLAStatsMonitor callback with Trainer that has no logger.")
 
         device = trainer.strategy.root_device
@@ -102,10 +102,11 @@ def on_train_epoch_end(self, trainer: "pl.Trainer", pl_module: "pl.LightningModu
         peak_memory = trainer.strategy.reduce(peak_memory) * 0.001
         epoch_time = trainer.strategy.reduce(epoch_time)
 
-        trainer.logger.log_metrics(
-            {"avg. free memory (MB)": float(free_memory), "avg. peak memory (MB)": float(peak_memory)},
-            step=trainer.current_epoch,
-        )
+        for logger in trainer.loggers:
+            logger.log_metrics(
+                {"avg. free memory (MB)": float(free_memory), "avg. peak memory (MB)": float(peak_memory)},
+                step=trainer.current_epoch,
+            )
 
         if self._verbose:
             rank_zero_info(f"Average Epoch time: {epoch_time:.2f} seconds")

pytorch_lightning/core/lightning.py

@@ -253,7 +253,7 @@ def logger(self) -> Optional[LightningLoggerBase]:
 
     @property
     def loggers(self) -> List[LightningLoggerBase]:
-        """Reference to the loggers object in the Trainer."""
+        """Reference to the list of loggers in the Trainer."""
         return self.trainer.loggers if self.trainer else []
 
     def _apply_batch_transfer_handler(

pytorch_lightning/loops/epoch/training_epoch_loop.py

@@ -504,8 +504,10 @@ def _save_loggers_on_train_batch_end(self) -> None:
         """Flushes loggers to disk."""
         # when loggers should save to disk
         should_flush_logs = self.trainer.logger_connector.should_flush_logs
-        if should_flush_logs and self.trainer.is_global_zero and self.trainer.logger is not None:
-            self.trainer.logger.save()
+        # TODO: is_global_zero check should be moved to logger.save() implementation
+        if should_flush_logs and self.trainer.is_global_zero:
+            for logger in self.trainer.loggers:
+                logger.save()
 
     def _reload_dataloader_state_dict(self, data_fetcher: AbstractDataFetcher) -> None:
         if self._dataloader_state_dict:

pytorch_lightning/plugins/precision/precision_plugin.py

@@ -155,7 +155,11 @@ def optimizer_step(
     def _track_grad_norm(self, trainer: "pl.Trainer") -> None:
         if trainer.track_grad_norm == -1:
             return
-        kwargs = {"group_separator": trainer.logger.group_separator} if trainer.logger is not None else {}
+
+        kwargs = {}
+        if len(trainer.loggers) == 1:
+            kwargs["group_separator"] = trainer.loggers[0].group_separator
+
         grad_norm_dict = grad_norm(trainer.lightning_module, trainer.track_grad_norm, **kwargs)
         if grad_norm_dict:
             prev_fx = trainer.lightning_module._current_fx_name

pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py

@@ -17,7 +17,7 @@
 
 import pytorch_lightning as pl
 from pytorch_lightning.accelerators import GPUAccelerator
-from pytorch_lightning.loggers import LightningLoggerBase, LoggerCollection, TensorBoardLogger
+from pytorch_lightning.loggers import LightningLoggerBase, TensorBoardLogger
 from pytorch_lightning.plugins.environments.slurm_environment import SLURMEnvironment
 from pytorch_lightning.trainer.connectors.logger_connector.result import _METRICS, _OUT_DICT, _PBAR_DICT
 from pytorch_lightning.trainer.states import RunningStage
@@ -90,15 +90,15 @@ def should_update_logs(self) -> bool:
     def configure_logger(self, logger: Union[bool, LightningLoggerBase, Iterable[LightningLoggerBase]]) -> None:
         if isinstance(logger, bool):
             # default logger
-            self.trainer.logger = (
-                TensorBoardLogger(save_dir=self.trainer.default_root_dir, version=SLURMEnvironment.job_id())
+            self.trainer.loggers = (
+                [TensorBoardLogger(save_dir=self.trainer.default_root_dir, version=SLURMEnvironment.job_id())]
                 if logger
-                else None
+                else []
             )
         elif isinstance(logger, Iterable):
-            self.trainer.logger = LoggerCollection(logger)
+            self.trainer.loggers = list(logger)
         else:
-            self.trainer.logger = logger
+            self.trainer.loggers = [logger]
 
     def log_metrics(self, metrics: _OUT_DICT, step: Optional[int] = None) -> None:
         """Logs the metric dict passed in. If `step` parameter is None and `step` key is presented is metrics, uses
@@ -109,7 +109,7 @@ def log_metrics(self, metrics: _OUT_DICT, step: Optional[int] = None) -> None:
             step: Step for which metrics should be logged. Default value is `self.global_step` during training or
                 the total validation / test log step count during validation and testing.
         """
-        if self.trainer.logger is None or not metrics:
+        if not self.trainer.loggers or not metrics:
             return
 
         self._logged_metrics.update(metrics)
@@ -126,11 +126,12 @@ def log_metrics(self, metrics: _OUT_DICT, step: Optional[int] = None) -> None:
             step = self.trainer.global_step
 
         # log actual metrics
-        if self._override_agg_and_log_metrics:
-            self.trainer.logger.agg_and_log_metrics(metrics=scalar_metrics, step=step)
-        else:
-            self.trainer.logger.log_metrics(metrics=scalar_metrics, step=step)
-        self.trainer.logger.save()
+        for logger in self.trainer.loggers:
+            if self._override_agg_and_log_metrics:
+                logger.agg_and_log_metrics(metrics=scalar_metrics, step=step)
+            else:
+                logger.log_metrics(metrics=scalar_metrics, step=step)
+            logger.save()
 
     """
     Evaluation metric updates

pytorch_lightning/trainer/connectors/signal_connector.py

@@ -66,8 +66,8 @@ def slurm_sigusr1_handler_fn(self, signum: _SIGNUM, frame: FrameType) -> None:
         rank_zero_info("handling SIGUSR1")
 
         # save logger to make sure we get all the metrics
-        if self.trainer.logger:
-            self.trainer.logger.finalize("finished")
+        for logger in self.trainer.loggers:
+            logger.finalize("finished")
         hpc_save_path = self.trainer._checkpoint_connector.hpc_save_path(self.trainer.weights_save_path)
         self.trainer.save_checkpoint(hpc_save_path)