# Create a weighted loss function to handle imbalance?
**URL:**
**Author:**  [@behlvaibhav](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Fu%2Fbehlvaibhav)%5C
**Post date:** [January 30, 2025, 6:19pm UTC](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fcreate-a-weighted-loss-function-to-handle-imbalance%2F138178%2F1 "2025-01-30T18:19:55Z")
Hi everyone,
I’m working on fine-tuning LLaMA 3.1 8B for a multi-class classification task using text data. However, my dataset is highly imbalanced, which is causing performance issues.
I have already tried several data augmentation techniques like back translation, paraphrasing, and contextual augmentation, but they didn’t significantly improve the balance since they mostly generated near-duplicate copies of the original text.
To address this, I decided to implement a weighted loss function (e.g., focal loss or class-weighted cross-entropy) to better handle class imbalance. However, I’m facing challenges in integrating this into the Hugging Face Trainer class.
Has anyone successfully implemented custom loss functions in Hugging Face’s Trainer? If so, could you guide me on the best way to modify the training loop or integrate focal loss with Trainer? Any help would be greatly appreciated!
Thanks in advance!
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**Author:**  [@John6666](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Fu%2FJohn6666)%5C
**Post date:** [January 31, 2025, 4:58am UTC](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fcreate-a-weighted-loss-function-to-handle-imbalance%2F138178%2F2 "2025-01-31T04:58:35Z")
There don’t seem to be many examples…
> [@SFTTrainer Loss function](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fsfttrainer-loss-function%2F80549)%3A
>
> I have a couple of questions: how to know the loss function used by default for SFTTrainer for a given model and how to alter it? For training an LLM the loss function is computed on the whole concatenated prompts, how to alter this and make loss function only compute on the output prompts
>
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**Author:**  [@Alanturner2](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Fu%2FAlanturner2)%5C
**Post date:** [February 1, 2025, 1:34am UTC](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fcreate-a-weighted-loss-function-to-handle-imbalance%2F138178%2F3 "2025-02-01T01:34:49Z")
Hi there!
I’ve faced a similar challenge before, so I thought I’d share what worked for me when using a custom loss (like focal loss) with Hugging Face’s Trainer.
### 1. Create a Custom Trainer
The easiest way to use your own loss function is to subclass the Trainer and override the `compute_loss` method. In this method, you can compute your loss (for example, focal loss or class-weighted cross-entropy) instead of the default loss. Here’s a simple example:
```python
from transformers import Trainer
class CustomLossTrainer(Trainer):
def __init__ (self, *args, loss_fn=None, **kwargs):
super(). __init__ (*args, **kwargs)
# Store your custom loss function.
# This should take (logits, labels) as arguments.
self.loss_fn = loss_fn
def compute_loss(self, model, inputs, return_outputs=False):
# Assume your inputs include "labels" and your model returns logits.
labels = inputs.get("labels")
outputs = model(**inputs)
logits = outputs.get("logits")
# Compute the custom loss using your loss function.
loss = self.loss_fn(logits, labels)
return (loss, outputs) if return_outputs else loss
```
### 2. Implement Your Custom Loss Function
You can define your custom loss function (for example, focal loss) as a separate function. For instance, here’s a simple version of focal loss using PyTorch:
```python
import torch
import torch.nn.functional as F
def focal_loss(logits, labels, gamma=2.0, alpha=0.25):
# Calculate standard cross-entropy loss first.
ce_loss = F.cross_entropy(logits, labels, reduction='none')
# Get softmax probabilities.
pt = torch.exp(-ce_loss)
# Compute focal loss.
focal_loss = alpha * (1 - pt) ** gamma * ce_loss
return focal_loss.mean()
```
### 3. Use the Custom Trainer
When you set up your training, pass your custom loss function to your trainer. For example:
```python
from transformers import TrainingArguments
training_args = TrainingArguments(
output_dir="./results",
num_train_epochs=3,
per_device_train_batch_size=8,
evaluation_strategy="epoch",
logging_steps=50,
save_steps=500,
)
trainer = CustomLossTrainer(
model=model,
args=training_args,
train_dataset=train_dataset,
eval_dataset=eval_dataset,
loss_fn=focal_loss, # Pass your custom loss function here.
)
trainer.train()
```
I hope this helps you integrate a custom loss function with Hugging Face’s Trainer and improves your model’s performance on imbalanced data. If you have any more questions or need further clarifications, feel free to ask!
Good luck, and happy fine-tuning!
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**Author:**  [@system](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Fu%2Fsystem)%5C
**Post date:** [May 21, 2025, 12:35pm UTC](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fcreate-a-weighted-loss-function-to-handle-imbalance%2F138178%2F5 "2025-05-21T12:35:44Z")
Closed on May 21, 2025, 12:35pm UTC
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