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().