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The lightweight PyTorch wrapper for ML researchers. Scale your models. Write less boilerplate
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PyTorch Lightning

The lightweight PyTorch wrapper for ML researchers. Scale your models. Write less boilerplate.

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Simple installation from PyPI

pip install pytorch-lightning  



View the docs here

What is it?

Lightning is a very lightweight wrapper on PyTorch. This means you don't have to learn a new library. To use Lightning, simply refactor your research code into the LightningModule format and Lightning will automate the rest. Lightning guarantees tested, correct, modern best practices for the automated parts.

How much effort is it to convert?

You're probably tired of switching frameworks at this point. But it is a very quick process to refactor into the Lightning format. Check out this tutorial

Starting a new project?

Use our seed-project aimed at reproducibility!

Why do I want to use lightning?

Every research project starts the same, a model, a training loop, validation loop, etc. As your research advances, you're likely to need distributed training, 16-bit precision, checkpointing, gradient accumulation, etc.

Lightning sets up all the boilerplate state-of-the-art training for you so you can focus on the research.

README Table of Contents

How do I do use it?

Think about Lightning as refactoring your research code instead of using a new framework. The research code goes into a LightningModule which you fit using a Trainer.

The LightningModule defines a system such as seq-2-seq, GAN, etc... It can ALSO define a simple classifier such as the example below.

To use lightning do 2 things:

  1. Define a LightningModule WARNING: This syntax is for version 0.5.0+ where abbreviations were removed.
    import os
    import torch
    from torch.nn import functional as F
    from import DataLoader
    from torchvision.datasets import MNIST
    from torchvision import transforms
    import pytorch_lightning as pl
    class CoolSystem(pl.LightningModule):
        def __init__(self):
            super(CoolSystem, self).__init__()
            # not the best model...
            self.l1 = torch.nn.Linear(28 * 28, 10)
        def forward(self, x):
            return torch.relu(self.l1(x.view(x.size(0), -1)))
        def training_step(self, batch, batch_idx):
            # REQUIRED
            x, y = batch
            y_hat = self.forward(x)
            loss = F.cross_entropy(y_hat, y)
            tensorboard_logs = {'train_loss': loss}
            return {'loss': loss, 'log': tensorboard_logs}
        def validation_step(self, batch, batch_idx):
            # OPTIONAL
            x, y = batch
            y_hat = self.forward(x)
            return {'val_loss': F.cross_entropy(y_hat, y)}
        def validation_end(self, outputs):
            # OPTIONAL
            avg_loss = torch.stack([x['val_loss'] for x in outputs]).mean()
            tensorboard_logs = {'val_loss': avg_loss}
            return {'avg_val_loss': avg_loss, 'log': tensorboard_logs}
        def configure_optimizers(self):
            # REQUIRED
            # can return multiple optimizers and learning_rate schedulers
            # (LBFGS it is automatically supported, no need for closure function)
            return torch.optim.Adam(self.parameters(), lr=0.02)
        def train_dataloader(self):
            # REQUIRED
            return DataLoader(MNIST(os.getcwd(), train=True, download=True, transform=transforms.ToTensor()), batch_size=32)
        def val_dataloader(self):
            # OPTIONAL
            return DataLoader(MNIST(os.getcwd(), train=True, download=True, transform=transforms.ToTensor()), batch_size=32)
        def test_dataloader(self):
            # OPTIONAL
            return DataLoader(MNIST(os.getcwd(), train=False, download=True, transform=transforms.ToTensor()), batch_size=32)
  2. Fit with a trainer
    from pytorch_lightning import Trainer
    model = CoolSystem()
    # most basic trainer, uses good defaults
    trainer = Trainer()   

Trainer sets up a tensorboard logger, early stopping and checkpointing by default (you can modify all of them or use something other than tensorboard).

Here are more advanced examples

# train on cpu using only 10% of the data (for demo purposes)
trainer = Trainer(max_num_epochs=1, train_percent_check=0.1)

# train on 4 gpus (lightning chooses GPUs for you)
# trainer = Trainer(max_num_epochs=1, gpus=4, distributed_backend='ddp')  

# train on 4 gpus (you choose GPUs)
# trainer = Trainer(max_num_epochs=1, gpus=[0, 1, 3, 7], distributed_backend='ddp')   

# train on 32 gpus across 4 nodes (make sure to submit appropriate SLURM job)
# trainer = Trainer(max_num_epochs=1, gpus=8, num_gpu_nodes=4, distributed_backend='ddp')

# train (1 epoch only here for demo)

# view tensorboard logs'View tensorboard logs by running\ntensorboard --logdir {os.getcwd()}')'and going to http://localhost:6006 on your browser')

When you're all done you can even run the test set separately.


What does lightning control for me?

Everything in gray!
You define the blue parts using the LightningModule interface:


# what to do in the training loop
def training_step(self, batch, batch_idx):

# what to do in the validation loop
def validation_step(self, batch, batch_idx):

# how to aggregate validation_step outputs
def validation_end(self, outputs):

# and your dataloaders
def train_dataloader():
def val_dataloader():
def test_dataloader():

Could be as complex as seq-2-seq + attention

# define what happens for training here
def training_step(self, batch, batch_idx):
    x, y = batch
    # define your own forward and loss calculation
    hidden_states = self.encoder(x)
    # even as complex as a seq-2-seq + attn model
    # (this is just a toy, non-working example to illustrate)
    start_token = '<SOS>'
    last_hidden = torch.zeros(...)
    loss = 0
    for step in range(max_seq_len):
        attn_context = self.attention_nn(hidden_states, start_token)
        pred = self.decoder(start_token, attn_context, last_hidden) 
        last_hidden = pred
        pred = self.predict_nn(pred)
        loss += self.loss(last_hidden, y[step])
    #toy example as well
    loss = loss / max_seq_len
    return {'loss': loss} 

Or as basic as CNN image classification

# define what happens for validation here
def validation_step(self, batch, batch_idx):    
    x, y = batch
    # or as basic as a CNN classification
    out = self.forward(x)
    loss = my_loss(out, y)
    return {'loss': loss} 

And you also decide how to collate the output of all validation steps

def validation_end(self, outputs):
    Called at the end of validation to aggregate outputs
    :param outputs: list of individual outputs of each validation step
    val_loss_mean = 0
    val_acc_mean = 0
    for output in outputs:
        val_loss_mean += output['val_loss']
        val_acc_mean += output['val_acc']

    val_loss_mean /= len(outputs)
    val_acc_mean /= len(outputs)
    logs = {'val_loss': val_loss_mean.item(), 'val_acc': val_acc_mean.item()}
    result = {'log': logs}
    return result


Lightning is fully integrated with tensorboard, MLFlow and supports any logging module.


Lightning also adds a text column with all the hyperparameters for this experiment.


Lightning automates all of the following (each is also configurable):


Computing cluster (SLURM)


Distributed training

Experiment Logging

Training loop

Validation loop

Testing loop



Asking for help

Welcome to the Lightning community!

If you have any questions, feel free to:

  1. read the docs.
  2. Search through the issues.
  3. Ask on stackoverflow with the tag pytorch-lightning.

If no one replies to you quickly enough, feel free to post the stackoverflow link to our Gitter chat!

To chat with the rest of us visit our gitter channel!


How do I use Lightning for rapid research?
Here's a walk-through

Why was Lightning created?
Lightning has 3 goals in mind:

  1. Maximal flexibility while abstracting out the common boilerplate across research projects.
  2. Reproducibility. If all projects use the LightningModule template, it will be much much easier to understand what's going on and where to look! It will also mean every implementation follows a standard format.
  3. Democratizing PyTorch power user features. Distributed training? 16-bit? know you need them but don't want to take the time to implement? All good... these come built into Lightning.

How does Lightning compare with Ignite and
Here's a thorough comparison.

Is this another library I have to learn?
Nope! We use pure Pytorch everywhere and don't add unecessary abstractions!

Are there plans to support Python 2?

Are there plans to support virtualenv?
Nope. Please use anaconda or miniconda.

Which PyTorch versions do you support?

  • PyTorch 1.1.0
    # install pytorch 1.1.0 using the official instructions   
    # install test-tube which supports 1.1.0   
    pip install test-tube==   
    # install latest Lightning version without upgrading deps    
    pip install -U --no-deps pytorch-lightning
  • PyTorch 1.2.0, 1.3.0, Install via pip as normal

Custom installation

Bleeding edge

If you can't wait for the next release, install the most up to date code with:

  • using GIT (locally clone whole repo with full history)
    pip install git+ --upgrade
  • using instant zip (last state of the repo without git history)
    pip install --upgrade

Any release installation

You can also install any past release from this repository:

pip install --upgrade


If you want to cite the framework feel free to use this (but only if you loved it 😊):

  author = {Falcon, W.A.},
  title = {PyTorch Lightning},
  year = {2019},
  publisher = {GitHub},
  journal = {GitHub repository},
  howpublished = {\url{}}
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