# Confused About Optimizer Behavior in Custom Training Loop

**URL:** <https://discuss.pytorch.org/t/confused-about-optimizer-behavior-in-custom-training-loop/219219>\
**Category:** Uncategorized\
**Created:** [April 18, 2025, 5:30am UTC](https://discuss.pytorch.org/t/confused-about-optimizer-behavior-in-custom-training-loop/219219 "2025-04-18T05:30:23Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![pasiho](https://discuss.pytorch.org/letter_avatar_proxy/v4/letter/p/a5b964/32.png) [@pasiho](https://discuss.pytorch.org/u/pasiho)\
**Post date:** [April 18, 2025, 5:30am UTC](https://discuss.pytorch.org/t/confused-about-optimizer-behavior-in-custom-training-loop/219219/1 "2025-04-18T05:30:23Z")

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Hey everyone! 👋

I’ve been experimenting with a custom training loop in PyTorch and stumbled upon a conceptual hurdle. I get the basics, but I want to understand better how the optimizer works during training.

Here’s the basic structure I’m using:

python  
Copy  
Edit  
for data target in loader:  
optimizer.zero\_grad()  
output = model(data)  
loss = criterion(output target)  
loss.backward()  
optimizer.step()

What I’m curious about: does optimizer.step() right away update the model weights using the gradients calculated from loss.backward()? And what happens if I change gradients in between—will those changes stick?

> [@Model Convergence Issue in Custom Training Loop](https://discuss.pytorch.org/t/model-convergence-issue-in-custom-training-loop/206714):
>
> There are issues with model convergence when using a custom training loop in PyTorch. The loss stagnates after a few epochs. The optimizer is Adam with a learning rate of 0.001. Issue persists even after adjusting the learning rate and changing the network architecture. Any advice on what might be causing this and potential solutions?

I’ve come across some posts here and there but would appreciate insights from those who have built custom training logic in more complex workflows.  
Also, I’m picking up knowledge about MLOps and automation. If you have any advice or suggestions for resources on [how to become a DevOps engineer](https://www.igmguru.com/blog/how-to-become-devops-engineer) while having deep learning experience, I’d be grateful!

Thanks in advance for your help!  
Regards  
pasiho

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<div class="post-metadata">

**Author:** ![ptrblck](https://discuss.pytorch.org/user_avatar/discuss.pytorch.org/ptrblck/32/1823_2.png) [@ptrblck](https://discuss.pytorch.org/u/ptrblck)\
**Post date:** [April 18, 2025, 12:30pm UTC](https://discuss.pytorch.org/t/confused-about-optimizer-behavior-in-custom-training-loop/219219/3 "2025-04-18T12:30:42Z")

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> [@pasiho](#):
>
> does optimizer.step() right away update the model weights using the gradients calculated from loss.backward()?

Yes, the `step()` method will use the `.grad` attributes of all trainable parameters which were passed to this optimizer to update them using the update formula (e.g. it could include momentum etc.).

> [@pasiho](#):
>
> And what happens if I change gradients in between—will those changes stick?

Yes, you can manipulate the gradients after the `backward` and before the `step()` call which is also a common practice for e.g. gradient clipping.