pytorch-lightning

PyTorch Lightning - High-Level Training Framework

Quick start

PyTorch Lightning organizes PyTorch code to eliminate boilerplate while maintaining flexibility.

Installation:

pip install lightning

Convert PyTorch to Lightning (3 steps):

import lightning as L
import torch
from torch import nn
from torch.utils.data import DataLoader, Dataset

# Step 1: Define LightningModule (organize your PyTorch code)
class LitModel(L.LightningModule):
    def __init__(self, hidden_size=128):
        super().__init__()
        self.model = nn.Sequential(
            nn.Linear(28 * 28, hidden_size),
            nn.ReLU(),
            nn.Linear(hidden_size, 10)
        )

    def training_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self.model(x)
        loss = nn.functional.cross_entropy(y_hat, y)
        self.log('train_loss', loss)  # Auto-logged to TensorBoard
        return loss

    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters(), lr=1e-3)

# Step 2: Create data
train_loader = DataLoader(train_dataset, batch_size=32)

# Step 3: Train with Trainer (handles everything else!)
trainer = L.Trainer(max_epochs=10, accelerator='gpu', devices=2)
model = LitModel()
trainer.fit(model, train_loader)

That's it! Trainer handles:

• GPU/TPU/CPU switching
• Distributed training (DDP, FSDP, DeepSpeed)
• Mixed precision (FP16, BF16)
• Gradient accumulation
• Checkpointing
• Logging
• Progress bars

Common workflows

Workflow 1: From PyTorch to Lightning

Original PyTorch code:

model = MyModel()
optimizer = torch.optim.Adam(model.parameters())
model.to('cuda')

for epoch in range(max_epochs):
    for batch in train_loader:
        batch = batch.to('cuda')
        optimizer.zero_grad()
        loss = model(batch)
        loss.backward()
        optimizer.step()

Lightning version:

class LitModel(L.LightningModule):
    def __init__(self):
        super().__init__()
        self.model = MyModel()

    def training_step(self, batch, batch_idx):
        loss = self.model(batch)  # No .to('cuda') needed!
        return loss

    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters())

# Train
trainer = L.Trainer(max_epochs=10, accelerator='gpu')
trainer.fit(LitModel(), train_loader)