Supported Loggers

The following are loggers we support:


Track your parameters, metrics, source code and more using Comet.


Log to local file system in yaml and CSV format.


Log using MLflow.


Log using Neptune.


Log to local or remote file system in TensorBoard format.


Log using Weights and Biases.

The above loggers will normally plot an additional chart (global_step VS epoch). Depending on the loggers you use, there might be some additional charts too.

By default, Lightning uses TensorBoard logger under the hood, and stores the logs to a directory (by default in lightning_logs/).

from lightning.pytorch import Trainer

# Automatically logs to a directory (by default ``lightning_logs/``)
trainer = Trainer()

To see your logs:

tensorboard --logdir=lightning_logs/

To visualize tensorboard in a jupyter notebook environment, run the following command in a jupyter cell:

%reload_ext tensorboard
%tensorboard --logdir=lightning_logs/

You can also pass a custom Logger to the Trainer.

from lightning.pytorch import loggers as pl_loggers

tb_logger = pl_loggers.TensorBoardLogger(save_dir="logs/")
trainer = Trainer(logger=tb_logger)

Choose from any of the others such as MLflow, Comet, Neptune, WandB, etc.

comet_logger = pl_loggers.CometLogger(save_dir="logs/")
trainer = Trainer(logger=comet_logger)

To use multiple loggers, simply pass in a list or tuple of loggers.

tb_logger = pl_loggers.TensorBoardLogger(save_dir="logs/")
comet_logger = pl_loggers.CometLogger(save_dir="logs/")
trainer = Trainer(logger=[tb_logger, comet_logger])


By default, Lightning logs every 50 steps. Use Trainer flags to Control Logging Frequency.


By default, all loggers log to os.getcwd(). You can change the logging path using Trainer(default_root_dir="/your/path/to/save/checkpoints") without instantiating a logger.

Logging from a LightningModule

Lightning offers automatic log functionalities for logging scalars, or manual logging for anything else.

Automatic Logging

Use the log() or log_dict() methods to log from anywhere in a LightningModule and callbacks.

def training_step(self, batch, batch_idx):
    self.log("my_metric", x)

# or a dict to log all metrics at once with individual plots
def training_step(self, batch, batch_idx):
    self.log_dict({"acc": acc, "recall": recall})


Everything explained below applies to both log() or log_dict() methods.

Depending on where the log() method is called, Lightning auto-determines the correct logging mode for you. Of course you can override the default behavior by manually setting the log() parameters.

def training_step(self, batch, batch_idx):
    self.log("my_loss", loss, on_step=True, on_epoch=True, prog_bar=True, logger=True)

The log() method has a few options:

  • on_step: Logs the metric at the current step.

  • on_epoch: Automatically accumulates and logs at the end of the epoch.

  • prog_bar: Logs to the progress bar (Default: False).

  • logger: Logs to the logger like Tensorboard, or any other custom logger passed to the Trainer (Default: True).

  • reduce_fx: Reduction function over step values for end of epoch. Uses torch.mean() by default and is not applied when a torchmetrics.Metric is logged.

  • enable_graph: If True, will not auto detach the graph.

  • sync_dist: If True, reduces the metric across devices. Use with care as this may lead to a significant communication overhead.

  • sync_dist_group: The DDP group to sync across.

  • add_dataloader_idx: If True, appends the index of the current dataloader to the name (when using multiple dataloaders). If False, user needs to give unique names for each dataloader to not mix the values.

  • batch_size: Current batch size used for accumulating logs logged with on_epoch=True. This will be directly inferred from the loaded batch, but for some data structures you might need to explicitly provide it.

  • rank_zero_only: Set this to True only if you call self.log explicitly only from rank 0. If True you won’t be able to access or specify this metric in callbacks (e.g. early stopping).

Default behavior of logging in Callback or LightningModule




on_train_start, on_train_epoch_start, on_train_epoch_end



on_before_backward, on_after_backward, on_before_optimizer_step, on_before_zero_grad



on_train_batch_start, on_train_batch_end, training_step



on_validation_start, on_validation_epoch_start, on_validation_epoch_end



on_validation_batch_start, on_validation_batch_end, validation_step




While logging tensor metrics with on_epoch=True inside step-level hooks and using mean-reduction (default) to accumulate the metrics across the current epoch, Lightning tries to extract the batch size from the current batch. If multiple possible batch sizes are found, a warning is logged and if it fails to extract the batch size from the current batch, which is possible if the batch is a custom structure/collection, then an error is raised. To avoid this, you can specify the batch_size inside the self.log(... batch_size=batch_size) call.

def training_step(self, batch, batch_idx):
    # extracts the batch size from `batch`
    self.log("train_loss", loss, on_epoch=True)

def validation_step(self, batch, batch_idx):
    # uses `batch_size=10`
    self.log("val_loss", loss, batch_size=10)


  • The above config for validation applies for test hooks as well.

  • Setting on_epoch=True will cache all your logged values during the full training epoch and perform a reduction in on_train_epoch_end. We recommend using TorchMetrics, when working with custom reduction.

  • Setting both on_step=True and on_epoch=True will create two keys per metric you log with suffix _step and _epoch respectively. You can refer to these keys e.g. in the monitor argument of ModelCheckpoint or in the graphs plotted to the logger of your choice.

If your work requires to log in an unsupported method, please open an issue with a clear description of why it is blocking you.

Manual Logging Non-Scalar Artifacts

If you want to log anything that is not a scalar, like histograms, text, images, etc., you may need to use the logger object directly.

def training_step(self):
    # the logger you used (in this case tensorboard)
    tensorboard = self.logger.experiment

Make a Custom Logger

You can implement your own logger by writing a class that inherits from Logger. Use the rank_zero_experiment() and rank_zero_only() decorators to make sure that only the first process in DDP training creates the experiment and logs the data respectively.

from lightning.pytorch.loggers.logger import Logger, rank_zero_experiment
from lightning.pytorch.utilities import rank_zero_only

class MyLogger(Logger):
    def name(self):
        return "MyLogger"

    def version(self):
        # Return the experiment version, int or str.
        return "0.1"

    def log_hyperparams(self, params):
        # params is an argparse.Namespace
        # your code to record hyperparameters goes here

    def log_metrics(self, metrics, step):
        # metrics is a dictionary of metric names and values
        # your code to record metrics goes here

    def save(self):
        # Optional. Any code necessary to save logger data goes here

    def finalize(self, status):
        # Optional. Any code that needs to be run after training
        # finishes goes here

If you write a logger that may be useful to others, please send a pull request to add it to Lightning!

Control Logging Frequency

Logging frequency

It may slow down training to log on every single batch. By default, Lightning logs every 50 rows, or 50 training steps. To change this behaviour, set the log_every_n_steps Trainer flag.

k = 10
trainer = Trainer(log_every_n_steps=k)

Log Writing Frequency

Individual logger implementations determine their flushing frequency. For example, on the CSVLogger you can set the flag flush_logs_every_n_steps.

Progress Bar

You can add any metric to the progress bar using log() method, setting prog_bar=True.

def training_step(self, batch, batch_idx):
    self.log("my_loss", loss, prog_bar=True)

You could learn more about progress bars supported by Lightning here.

Modifying the Progress Bar

The progress bar by default already includes the training loss and version number of the experiment if you are using a logger. These defaults can be customized by overriding the get_metrics() hook in your logger.

from lightning.pytorch.callbacks.progress import TQDMProgressBar

class CustomProgressBar(TQDMProgressBar):
    def get_metrics(self, *args, **kwargs):
        # don't show the version number
        items = super().get_metrics(*args, **kwargs)
        items.pop("v_num", None)
        return items

Configure Console Logging

Lightning logs useful information about the training process and user warnings to the console. You can retrieve the Lightning console logger and change it to your liking. For example, adjust the logging level or redirect output for certain modules to log files:

import logging

# configure logging at the root level of Lightning

# configure logging on module level, redirect to file
logger = logging.getLogger("lightning.pytorch.core")

Read more about custom Python logging here.

Logging Hyperparameters

When training a model, it is useful to know what hyperparams went into that model. When Lightning creates a checkpoint, it stores a key "hyper_parameters" with the hyperparams.

lightning_checkpoint = torch.load(filepath, map_location=lambda storage, loc: storage)
hyperparams = lightning_checkpoint["hyper_parameters"]

Some loggers also allow logging the hyperparams used in the experiment. For instance, when using the TensorBoardLogger, all hyperparams will show in the hparams tab at torch.utils.tensorboard.writer.SummaryWriter.add_hparams().


If you want to track a metric in the tensorboard hparams tab, log scalars to the key hp_metric. If tracking multiple metrics, initialize TensorBoardLogger with default_hp_metric=False and call log_hyperparams only once with your metric keys and initial values. Subsequent updates can simply be logged to the metric keys. Refer to the examples below for setting up proper hyperparams metrics tracking within the LightningModule.

# Using default_hp_metric
def validation_step(self, batch, batch_idx):
    self.log("hp_metric", some_scalar)

# Using custom or multiple metrics (default_hp_metric=False)
def on_train_start(self):
    self.logger.log_hyperparams(self.hparams, {"hp/metric_1": 0, "hp/metric_2": 0})

def validation_step(self, batch, batch_idx):
    self.log("hp/metric_1", some_scalar_1)
    self.log("hp/metric_2", some_scalar_2)

In the example, using "hp/" as a prefix allows for the metrics to be grouped under “hp” in the tensorboard scalar tab where you can collapse them.

Managing Remote Filesystems

Lightning supports saving logs to a variety of filesystems, including local filesystems and several cloud storage providers.

Check out the Remote Filesystems doc for more info.