transformers/pr_26617/en/data_collators.md

# Data collators

A data collator assembles individual dataset samples into a batch for the model. It can also dynamically pad samples to the longest sequence in *each batch*, which is more efficient than padding to a global maximum length.

```md
Dataset[0] → {"input_ids": [101, 2003], "labels": 1}
Dataset[1] → {"input_ids": [101, 2003, 1996], "labels": 0}
Dataset[2] → {"input_ids": [101, 7592], "labels": 1}
         ↓  collator
{
  "input_ids": tensor([[101, 2003,    0],   # padded to longest
                        [101, 2003, 1996],
                        [101, 7592,    0]]),
  "labels":    tensor([1, 0, 1])
}
```

Transformers provides data collators for various tasks (see all available [data collators](./main_classes/data_collator)). Create a custom data collator with:

- [DataCollatorWithPadding](#datacollatorwithpadding) when you need standard tokenizer-based padding plus extra fields.
- [DataCollatorMixin](#datacollatormixin) when you need custom padding logic, multiple paired inputs per sample, or a batch structure the tokenizer can't produce on its own.

## DataCollatorWithPadding

For simple use cases like adding an extra field, subclass [DataCollatorWithPadding](/docs/transformers/pr_26617/en/main_classes/data_collator#transformers.DataCollatorWithPadding) and extend its `__call__` method. The example below adds a `"score"` field.

1. Remove the custom field first because [pad()](/docs/transformers/pr_26617/en/internal/tokenization_utils#transformers.PreTrainedTokenizerBase.pad) doesn't recognize it.
2. Call the parent class to handle `input_ids` and `attention_mask`.
3. Add the `"score"` field back to the batch.

```py
import torch
from dataclasses import dataclass
from transformers import DataCollatorWithPadding, PreTrainedTokenizerBase

@dataclass
class DataCollatorWithScore(DataCollatorWithPadding):
    tokenizer: PreTrainedTokenizerBase

    def __call__(self, features):
        scores = [f.pop("score") for f in features]

        batch = super().__call__(features)
        batch["score"] = torch.tensor(scores, dtype=torch.float)

        return batch
```

Pass the custom data collator to [Trainer](/docs/transformers/pr_26617/en/main_classes/trainer#transformers.Trainer) like any other data collator.

```py
trainer = Trainer(
    ...,
    data_collator=DataCollatorWithScore(tokenizer=tokenizer),
)
```

## DataCollatorMixin

Subclass `DataCollatorMixin` for full control over batch assembly and implement your own `__call__` method. Build custom padding logic, handle multiple input types, or create entirely new batch structures. The [DataCollatorForPreference](https://github.com/huggingface/trl/blob/cfbdd3bea4448cde878c0da0de49551f553c61fe/trl/trainer/reward_trainer.py#L126) example below uses `DataCollatorMixin` because each training sample has a chosen and rejected response, and the model needs to see both.

1. Separate `chosen_ids` and `rejected_ids` because `pad` expects flat lists.
2. Concatenate the input pair into a single list.
3. Generate `attention_mask` with [torch.ones_like](https://docs.pytorch.org/docs/stable/generated/torch.ones_like.html) instead of the tokenizer because the collator works with raw token ID lists.
4. Pad `input_ids` and `attention_mask`.

```py
import torch
from transformers import DataCollatorMixin
from trl.trainer.utils import pad

class DataCollatorForPreference(DataCollatorMixin):
    pad_token_id: int
    pad_to_multiple_of: int | None = None

    def __call__(self, examples: list[dict]) -> dict:
        chosen_input_ids   = [torch.tensor(ex["chosen_ids"])   for ex in examples]
        rejected_input_ids = [torch.tensor(ex["rejected_ids"]) for ex in examples]

        input_ids      = chosen_input_ids + rejected_input_ids
        attention_mask = [torch.ones_like(ids) for ids in input_ids]

        output = {
            "input_ids": pad(
                input_ids,
                padding_value=self.pad_token_id,
                padding_side="right",
                pad_to_multiple_of=self.pad_to_multiple_of,
            ),
            "attention_mask": pad(
                attention_mask,
                padding_value=0,
                padding_side="right",
                pad_to_multiple_of=self.pad_to_multiple_of,
            ),
        }

        ...

        return output
```

## Next steps

- See all available [data collators](./main_classes/data_collator) for common tasks like token classification.