Pytorch Patterns

---
name: pytorch-patterns
description: PyTorch deep learning patterns and best practices for building robust, efficient, and reproducible training pipelines, model architectures, and data loading. Use when writing or reviewing PyTorch training loops, model architectures, or data loading, or when a run will not reproduce.
metadata:
  origin: ECC
---

# PyTorch Development Patterns

Idiomatic PyTorch patterns and best practices for building robust, efficient, and reproducible deep learning applications.

## When to Activate

- Writing new PyTorch models or training scripts
- Reviewing deep learning code
- Debugging training loops or data pipelines
- Optimizing GPU memory usage or training speed
- Setting up reproducible experiments

## Core Principles

### 1. Device-Agnostic Code

Always write code that works on both CPU and GPU without hardcoding devices.

```python
# Good: Device-agnostic
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
model = MyModel().to(device)
data = data.to(device)

# Bad: Hardcoded device
model = MyModel().cuda()  # Crashes if no GPU
data = data.cuda()
```

### 2. Reproducibility First

Set all random seeds for reproducible results.

```python
# Good: Full reproducibility setup
def set_seed(seed: int = 42) -> None:
    torch.manual_seed(seed)
    torch.cuda.manual_seed_all(seed)
    np.random.seed(seed)
    random.seed(seed)
    torch.backends.cudnn.deterministic = True
    torch.backends.cudnn.benchmark = False

# Bad: No seed control
model = MyModel()  # Different weights every run
```

### 3. Explicit Shape Management

Always document and verify tensor shapes.

```python
# Good: Shape-annotated forward pass
def forward(self, x: torch.Tensor) -> torch.Tensor:
    # x: (batch_size, channels, height, width)
    x = self.conv1(x)    # -> (batch_size, 32, H, W)
    x = self.pool(x)     # -> (batch_size, 32, H//2, W//2)
    x = x.view(x.size(0), -1)  # -> (batch_size, 32*H//2*W//2)
    return self.fc(x)    # -> (batch_size, num_classes)

# Bad: No shape tracking
def forward(self, x):
    x = self.conv1(x)
    x = self.pool(x)
    x = x.view(x.size(0), -1)  # What size is this?
    return self.fc(x)           # Will this even work?
```

## Model Architecture Patterns

### Clean nn.Module Structure

```python
# Good: Well-organized module
class ImageClassifier(nn.Module):
    def __init__(self, num_classes: int, dropout: float = 0.5) -> None:
        super().__init__()
        self.features = nn.Sequential(
            nn.Conv2d(3, 64, kernel_size=3, padding=1),
            nn.BatchNorm2d(64),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(2),
        )
        self.classifier = nn.Sequential(
            nn.Dropout(dropout),
            nn.Linear(64 * 16 * 16, num_classes),
        )

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        x = self.features(x)
        x = x.view(x.size(0), -1)
        return self.classifier(x)

# Bad: Everything in forward
class ImageClassifier(nn.Module):
    def __init__(self):
        super().__init__()

    def forward(self, x):
        x = F.conv2d(x, weight=self.make_weight())  # Creates weight each call!
        return x
```

### Proper Weight Initialization

```python
# Good: Explicit initialization
def _init_weights(self, module: nn.Module) -> None:
    if isinstance(module, nn.Linear):
        nn.init.kaiming_normal_(module.weight, mode="fan_out", nonlinearity="relu")
        if module.bias is not None:
            nn.init.zeros_(module.bias)
    elif isinstance(module, nn.Conv2d):
        nn.init.kaiming_normal_(module.weight, mode="fan_out", nonlinearity="relu")
    elif isinstance(module, nn.BatchNorm2d):
        nn.init.ones_(module.weight)
        nn.init.zeros_(module.bias)

model = MyModel()
model.apply(model._init_weights)
```

## Training Loop Patterns

### Standard Training Loop

```python
# Good: Complete training loop with best practices
def train_one_epoch(
    model: nn.Module,
    dataloader: DataLoader,
    optimizer: torch.optim.Optimizer,
    criterion: nn.Module,
    device: torch.device,
    scaler: torch.amp.GradScaler | None = None,
) -> float:
    model.train()  # Always set train mode
    total_loss = 0.0

    for batch_idx, (data, target) in enumerate(dataloader):
        data, target = data.to(device), target.to(device)

        optimizer.zero_grad(set_to_none=True)  # More efficient than zero_grad()

        # Mixed precision training
        with torch.amp.autocast("cuda", enabled=scaler is not None):
            output = model(data)
            loss = criterion(output, target)

        if scaler is not None:
            scaler.scale(loss).backward()
            scaler.unscale_(optimizer)
            torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1.0)
            scaler.step(optimizer)
            scaler.update()
        else:
            loss.backward()
            torch.nn.utils.clip_grad_norm_(model.parameters(), max_norm=1.0)
            optimizer.step()

        total_loss += loss.item()

    return total_loss / len(dataloader)
```

### Validation Loop

```python
# Good: Proper evaluation
@torch.no_grad()  # More efficient than wrapping in torch.no_grad() block
def evaluate(
    model: nn.Module,
    dataloader: DataLoader,
    criterion: nn.Module,
    device: torch.device,
) -> tuple[float, float]:
    model.eval()  # Always set eval mode — disables dropout, uses running BN stats
    total_loss = 0.0
    correct = 0
    total = 0

    for data, target in dataloader:
        data, target = data.to(device), target.to(device)
        output = model(data)
        total_loss += criterion(output, target).item()
        correct += (output.argmax(1) == target).sum().item()
        total += target.size(0)

    return total_loss / len(dataloader), correct / total
```

## Data Pipeline Patterns

### Custom Dataset

```python
# Good: Clean Dataset with type hints
class ImageDataset(Dataset):
    def __init__(
        self,
        image_dir: str,
        labels: dict[str, int],
        transform: transforms.Compose | None = None,
    ) -> None:
        self.image_paths = list(Path(image_dir).glob("*.jpg"))
        self.labels = labels
        self.transform = transform

    def __len__(self) -> int:
        return len(self.image_paths)

    def __getitem__(self, idx: int) -> tuple[torch.Tensor, int]:
        img = Image.open(self.image_paths[idx]).convert("RGB")
        label = self.labels[self.image_paths[idx].stem]

        if self.transform:
            img = self.transform(img)

        return img, label
```

### Efficient DataLoader Configuration

```python
# Good: Optimized DataLoader
dataloader = DataLoader(
    dataset,
    batch_size=32,
    shuffle=True,            # Shuffle for training
    num_workers=4,           # Parallel data loading
    pin_memory=True,         # Faster CPU->GPU transfer
    persistent_workers=True, # Keep workers alive between epochs
    drop_last=True,          # Consistent batch sizes for BatchNorm
)

# Bad: Slow defaults
dataloader = DataLoader(dataset, batch_size=32)  # num_workers=0, no pin_memory
```

### Custom Collate for Variable-Length Data

```python
# Good: Pad sequences in collate_fn
def collate_fn(batch: list[tuple[torch.Tensor, int]]) -> tuple[torch.Tensor, torch.Tensor]:
    sequences, labels = zip(*batch)
    # Pad to max length in batch
    padded = nn.utils.rnn.pad_sequence(sequences, batch_first=True, padding_value=0)
    return padded, torch.tensor(labels)

dataloader = DataLoader(dataset, batch_size=32, collate_fn=collate_fn)
```

## Checkpointing Patterns

### Save and Load Checkpoints

```python
# Good: Complete checkpoint with all training state
def save_checkpoint(
    model: nn.Module,
    optimizer: torch.optim.Optimizer,
    epoch: int,
    loss: float,
    path: str,
) -> None:
    torch.save({
        "epoch": epoch,
        "model_state_dict": model.state_dict(),
        "optimizer_state_dict": optimizer.state_dict(),
        "loss": loss,
    }, path)

def load_checkpoint(
    path: str,
    model: nn.Module,
    optimizer: torch.optim.Optimizer | None = None,
) -> dict:
    checkpoint = torch.load(path, map_location="cpu", weights_only=True)
    model.load_state_dict(checkpoint["model_state_dict"])
    if optimizer:
        optimizer.load_state_dict(checkpoint["optimizer_state_dict"])
    return checkpoint

# Bad: Only saving model weights (can't resume training)
torch.save(model.state_dict(), "model.pt")
```

## Performance Optimization

### Mixed Precision Training

```python
# Good: AMP with GradScaler
scaler = torch.amp.GradScaler("cuda")
for data, target in dataloader:
    with torch.amp.autocast("cuda"):
        output = model(data)
        loss = criterion(output, target)
    scaler.scale(loss).backward()
    scaler.step(optimizer)
    scaler.update()
    optimizer.zero_grad(set_to_none=True)
```

### Gradient Checkpointing for Large Models

```python
# Good: Trade compute for memory
from torch.utils.checkpoint import checkpoint

class LargeModel(nn.Module):
    def forward(self, x: torch.Tensor) -> torch.Tensor:
        # Recompute activations during backward to save memory
        x = checkpoint(self.block1, x, use_reentrant=False)
        x = checkpoint(self.block2, x, use_reentrant=False)
        return self.head(x)
```

### torch.compile for Speed

```python
# Good: Compile the model for faster execution (PyTorch 2.0+)
model = MyModel().to(device)
model = torch.compile(model, mode="reduce-overhead")

# Modes: "default" (safe), "reduce-overhead" (faster), "max-autotune" (fastest)
```

## Quick Reference: PyTorch Idioms

| Idiom | Description |
|-------|-------------|
| `model.train()` / `model.eval()` | Always set mode before train/eval |
| `torch.no_grad()` | Disable gradients for inference |
| `optimizer.zero_grad(set_to_none=True)` | More efficient gradient clearing |
| `.to(device)` | Device-agnostic tensor/model placement |
| `torch.amp.autocast` | Mixed precision for 2x speed |
| `pin_memory=True` | Faster CPU→GPU data transfer |
| `torch.compile` | JIT compilation for speed (2.0+) |
| `weights_only=True` | Secure model loading |
| `torch.manual_seed` | Reproducible experiments |
| `gradient_checkpointing` | Trade compute for memory |

## Anti-Patterns to Avoid

```python
# Bad: Forgetting model.eval() during validation
model.train()
with torch.no_grad():
    output = model(val_data)  # Dropout still active! BatchNorm uses batch stats!

# Good: Always set eval mode
model.eval()
with torch.no_grad():
    output = model(val_data)

# Bad: In-place operations breaking autograd
x = F.relu(x, inplace=True)  # Can break gradient computation
x += residual                  # In-place add breaks autograd graph

# Good: Out-of-place operations
x = F.relu(x)
x = x + residual

# Bad: Moving data to GPU inside the training loop repeatedly
for data, target in dataloader:
    model = model.cuda()  # Moves model EVERY iteration!

# Good: Move model once before the loop
model = model.to(device)
for data, target in dataloader:
    data, target = data.to(device), target.to(device)

# Bad: Using .item() before backward
loss = criterion(output, target).item()  # Detaches from graph!
loss.backward()  # Error: can't backprop through .item()

# Good: Call .item() only for logging
loss = criterion(output, target)
loss.backward()
print(f"Loss: {loss.item():.4f}")  # .item() after backward is fine

# Bad: Not using torch.save properly
torch.save(model, "model.pt")  # Saves entire model (fragile, not portable)

# Good: Save state_dict
torch.save(model.state_dict(), "model.pt")
```

__Remember__: PyTorch code should be device-agnostic, reproducible, and memory-conscious. When in doubt, profile with `torch.profiler` and check GPU memory with `torch.cuda.memory_summary()`.