Phase 00 - Lesson 12

Debugging and Profiling

The worst AI bugs don't crash. They train silently on garbage and report a beautiful loss curve.

Type: Build Language: Python Prerequisites: Lesson 1 (Dev Environment), basic PyTorch familiarity Time: ~60 minutes

Learning Objectives

Use conditional breakpoint() and debug_print to inspect tensor shapes, dtypes, and NaN values mid-training
Profile training loops with cProfile, line_profiler, and tracemalloc to find bottlenecks
Detect common AI bugs: shape mismatches, NaN loss, data leakage, and wrong-device tensors
Set up TensorBoard to visualize loss curves, weight histograms, and gradient distributions

The Problem

AI code fails differently than regular code. A web app crashes with a stack trace. A misconfigured training loop runs for 8 hours, burns

00 in GPU time, and produces a model that predicts the mean of every input. The code never errored. The bug was a tensor on the wrong device, a forgotten .detach(), or labels leaking into features.

You need debugging tools that catch these silent failures before they waste your time and compute.

The Concept

AI debugging operates at three levels:

graph TD
    L3["3. Training Dynamics<br/>Loss curves, gradient norms, activations"] --> L2
    L2["2. Tensor Operations<br/>Shapes, dtypes, devices, NaN/Inf values"] --> L1
    L1["1. Standard Python<br/>Breakpoints, logging, profiling, memory"]

Most people jump straight to level 3 (staring at TensorBoard). But 80% of AI bugs live at levels 1 and 2.

Build It

Part 1: Print Debugging (Yes, It Works)

Print debugging gets dismissed. It shouldn't. For tensor code, a targeted print statement beats stepping through a debugger because you need to see shapes, dtypes, and value ranges all at once.

def debug_print(name, tensor):
    print(f"{name}: shape={tensor.shape}, dtype={tensor.dtype}, "
          f"device={tensor.device}, "
          f"min={tensor.min().item():.4f}, max={tensor.max().item():.4f}, "
          f"mean={tensor.mean().item():.4f}, "
          f"has_nan={tensor.isnan().any().item()}")

Call this after every suspicious operation. When the bug is found, remove the prints. Simple.

Part 2: Python Debugger (pdb and breakpoint)

The built-in debugger is underrated for AI work. Drop breakpoint() into your training loop and inspect tensors interactively.

def training_step(model, batch, criterion, optimizer):
    inputs, labels = batch
    outputs = model(inputs)
    loss = criterion(outputs, labels)

    if loss.item() > 100 or torch.isnan(loss):
        breakpoint()

    loss.backward()
    optimizer.step()

When the debugger drops you in, useful commands:

p outputs.shape to check shapes
p loss.item() to see the loss value
p torch.isnan(outputs).sum() to count NaNs
p model.fc1.weight.grad to check gradients
c to continue, q to quit

This is conditional debugging. You only stop when something looks wrong. For a 10,000-step training run, that matters.

Part 3: Python Logging

Replace print statements with logging when your debugging goes beyond a quick check.

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s [%(levelname)s] %(message)s",
    handlers=[
        logging.FileHandler("training.log"),
        logging.StreamHandler()
    ]
)
logger = logging.getLogger(__name__)

logger.info("Starting training: lr=%.4f, batch_size=%d", lr, batch_size)
logger.warning("Loss spike detected: %.4f at step %d", loss.item(), step)
logger.error("NaN loss at step %d, stopping", step)

Logging gives you timestamps, severity levels, and file output. When a training run fails at 3 AM, you want a log file, not terminal output that scrolled off screen.

Part 4: Timing Code Sections

Knowing where time goes is the first step to optimization.

import time

class Timer:
    def __init__(self, name=""):
        self.name = name

    def __enter__(self):
        self.start = time.perf_counter()
        return self

    def __exit__(self, *args):
        elapsed = time.perf_counter() - self.start
        print(f"[{self.name}] {elapsed:.4f}s")

with Timer("data loading"):
    batch = next(dataloader_iter)

with Timer("forward pass"):
    outputs = model(batch)

with Timer("backward pass"):
    loss.backward()

Common finding: data loading takes 60% of training time. The fix is num_workers > 0 in your DataLoader, not a faster GPU.

Part 5: cProfile and line_profiler

When you need more than manual timers:

python -m cProfile -s cumtime train.py

This shows every function call sorted by cumulative time. For line-by-line profiling:

pip install line_profiler

@profile
def train_step(model, data, target):
    output = model(data)
    loss = F.cross_entropy(output, target)
    loss.backward()
    return loss

# Run with: kernprof -l -v train.py

Part 6: Memory Profiling

CPU Memory with tracemalloc

import tracemalloc

tracemalloc.start()

# your code here
model = build_model()
data = load_dataset()

snapshot = tracemalloc.take_snapshot()
top_stats = snapshot.statistics("lineno")
for stat in top_stats[:10]:
    print(stat)

CPU Memory with memory_profiler

pip install memory_profiler

from memory_profiler import profile

@profile
def load_data():
    raw = read_csv("data.csv")       # watch memory jump here
    processed = preprocess(raw)       # and here
    return processed

Run with python -m memory_profiler your_script.py to see line-by-line memory usage.

GPU Memory with PyTorch

import torch

if torch.cuda.is_available():
    print(torch.cuda.memory_summary())

    print(f"Allocated: {torch.cuda.memory_allocated() / 1e9:.2f} GB")
    print(f"Cached: {torch.cuda.memory_reserved() / 1e9:.2f} GB")

When you hit OOM (Out of Memory):

Reduce batch size (first thing to try, always)
Use torch.cuda.empty_cache() to free cached memory
Use del tensor followed by torch.cuda.empty_cache() for large intermediates
Use mixed precision (torch.cuda.amp) to halve memory usage
Use gradient checkpointing for very deep models

Part 7: Common AI Bugs and How to Catch Them

Shape Mismatch

The most frequent bug. A tensor has shape [batch, features] when the model expects [batch, channels, height, width].

def check_shapes(model, sample_input):
    print(f"Input: {sample_input.shape}")
    hooks = []

    def make_hook(name):
        def hook(module, inp, out):
            in_shape = inp[0].shape if isinstance(inp, tuple) else inp.shape
            out_shape = out.shape if hasattr(out, "shape") else type(out)
            print(f"  {name}: {in_shape} -> {out_shape}")
        return hook

    for name, module in model.named_modules():
        hooks.append(module.register_forward_hook(make_hook(name)))

    with torch.no_grad():
        model(sample_input)

    for h in hooks:
        h.remove()

Run this once with a sample batch. It maps every shape transformation in your model.

NaN Loss

NaN loss means something exploded. Common causes:

Learning rate too high
Division by zero in custom loss
Log of zero or negative number
Exploding gradients in RNNs

def detect_nan(model, loss, step):
    if torch.isnan(loss):
        print(f"NaN loss at step {step}")
        for name, param in model.named_parameters():
            if param.grad is not None:
                if torch.isnan(param.grad).any():
                    print(f"  NaN gradient in {name}")
                if torch.isinf(param.grad).any():
                    print(f"  Inf gradient in {name}")
        return True
    return False

Data Leakage

Your model gets 99% accuracy on the test set. Sounds great. It's a bug.

def check_data_leakage(train_set, test_set, id_column="id"):
    train_ids = set(train_set[id_column].tolist())
    test_ids = set(test_set[id_column].tolist())
    overlap = train_ids & test_ids
    if overlap:
        print(f"DATA LEAKAGE: {len(overlap)} samples in both train and test")
        return True
    return False

Also check for temporal leakage: using future data to predict the past. Sort by timestamp before splitting.

Wrong Device

Tensors on different devices (CPU vs GPU) cause runtime errors. But sometimes a tensor silently stays on CPU while everything else is on GPU, and training just runs slowly.

def check_devices(model, *tensors):
    model_device = next(model.parameters()).device
    print(f"Model device: {model_device}")
    for i, t in enumerate(tensors):
        if t.device != model_device:
            print(f"  WARNING: tensor {i} on {t.device}, model on {model_device}")

Part 8: TensorBoard Basics

TensorBoard shows you what's happening inside training over time.

pip install tensorboard

from torch.utils.tensorboard import SummaryWriter

writer = SummaryWriter("runs/experiment_1")

for step in range(num_steps):
    loss = train_step(model, batch)

    writer.add_scalar("loss/train", loss.item(), step)
    writer.add_scalar("lr", optimizer.param_groups[0]["lr"], step)

    if step % 100 == 0:
        for name, param in model.named_parameters():
            writer.add_histogram(f"weights/{name}", param, step)
            if param.grad is not None:
                writer.add_histogram(f"grads/{name}", param.grad, step)

writer.close()

Launch it:

tensorboard --logdir=runs

What to look for:

Loss not decreasing: Learning rate too low, or model architecture issue
Loss oscillating wildly: Learning rate too high
Loss goes to NaN: Numerical instability (see NaN section above)
Train loss decreasing, val loss increasing: Overfitting
Weight histograms collapsing to zero: Vanishing gradients
Gradient histograms exploding: Need gradient clipping

Part 9: VS Code Debugger

For interactive debugging, configure VS Code with a launch.json:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Debug Training",
            "type": "debugpy",
            "request": "launch",
            "program": "${file}",
            "console": "integratedTerminal",
            "justMyCode": false
        }
    ]
}

Set breakpoints by clicking the gutter. Use the Variables pane to inspect tensor properties. The Debug Console lets you run arbitrary Python expressions mid-execution.

Useful for stepping through data preprocessing pipelines where you want to see each transformation.

Use It

Here's the debugging workflow that catches most AI bugs:

Before training: Run check_shapes with a sample batch. Verify input and output dimensions match expectations.
First 10 steps: Use debug_print on loss, outputs, and gradients. Confirm nothing is NaN and values are in reasonable ranges.
During training: Log loss, learning rate, and gradient norms. Use TensorBoard for visualization.
When something breaks: Drop breakpoint() at the failure point. Inspect tensors interactively.
For performance: Time your data loading vs forward vs backward pass. Profile memory if you're near OOM.

Ship It

Run the debugging toolkit script:

python phases/00-setup-and-tooling/12-debugging-and-profiling/code/debug_tools.py

See outputs/prompt-debug-ai-code.md for a prompt that helps diagnose AI-specific bugs.

Exercises

Run debug_tools.py and read through each section's output. Modify the dummy model to introduce a NaN (hint: divide by zero in the forward pass) and watch the detector catch it.
Profile a training loop with cProfile and identify the slowest function.
Use tracemalloc to find which line in your data loading pipeline allocates the most memory.
Set up TensorBoard for a simple training run and identify whether the model is overfitting.
Use breakpoint() inside a training loop. Practice inspecting tensor shapes, devices, and gradient values from the debugger prompt.