Write a Minimal Training Loop
Loop over data and update weights.
Write a Minimal Training Loop is a free Deep Learning Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Deep Learning Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
From Steps to a Loop
You know the four moves of one iteration. Now you wrap them in a loop that walks over your data again and again until the model is trained. 🔁
Gather the Ingredients
Before looping you need three things: a model, a loss function, and an optimizer that holds your model's parameters. Set them up once, up front.
loss_fn = nn.CrossEntropyLoss()
optimizer = torch.optim.Adam(model.parameters())Loop Over Epochs
The outer loop counts epochs, one full pass through your training data. A few epochs may be plenty for a small problem, more for a hard one.
for epoch in range(epochs):
...Loop Over Batches
Inside each epoch you iterate the dataloader, which hands you one batch of inputs and labels at a time instead of the whole dataset at once.
for x, y in dataloader:
...Zero, Then Forward
Start each batch by clearing old gradients, then run the forward pass to get predictions. Order matters: zero first, then predict.
optimizer.zero_grad()
pred = model(x)Measure, Then Learn
Compute the loss, call backward for gradients, then step the optimizer. These three lines are where every weight actually improves.
loss = loss_fn(pred, y)
loss.backward()
optimizer.step()The Full Skeleton
Stitch it together and you have a complete training loop. It is short on purpose: this same shape powers projects of every size.
for epoch in range(epochs):
for x, y in dataloader:
optimizer.zero_grad()
loss = loss_fn(model(x), y)
loss.backward()
optimizer.step()Watch the Loss
Print the loss each epoch so you can see learning happen. A number that trends down means your loop is working as intended.
print(epoch, loss.item())Why item Matters
Use loss.item to pull out a plain Python float. Logging the raw tensor instead keeps the whole computation graph alive and wastes memory.
If Loss Stalls
If the loss refuses to drop, suspect the basics first: a learning rate that is too high or too low, or a forgotten zero_grad each step.
Small but Complete
This loop is tiny yet it is genuinely complete. Everything fancier you meet later just adds validation, logging, or speed on top of these few lines.
Quick Check
What does the inner loop iterate over in a minimal training loop?
Recap
You built a real training loop: loop epochs, loop batches, then zero, forward, loss, backward, step. Print the loss to watch it learn. 🎉
Frequently asked questions
Is the “Write a Minimal Training Loop” lesson free?
Yes — the full text of “Write a Minimal Training Loop” is free to read here on the web, and the Deep Learning Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Deep Learning Academy course, upgrade to CoddyKit PRO.
What will I learn in “Write a Minimal Training Loop”?
Loop over data and update weights. You practise Deep Learning Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Deep Learning Academy?
No prior experience is required. Deep Learning Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Write a Minimal Training Loop” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Deep Learning Academy lesson?
Yes. Every Deep Learning Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Forward Pass, Loss, Backward, Step
- Write a Minimal Training Loop
- Track Accuracy While You Train
- Train vs Eval Mode