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.

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). Create a custom data collator with:

DataCollatorWithPadding when you need standard tokenizer-based padding plus extra fields.
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] and extend its __call__ method. The example below adds a "score" field.

Remove the custom field first because [~PreTrainedTokenizerBase.pad] doesn't recognize it.
Call the parent class to handle input_ids and attention_mask.
Add the "score" field back to the batch.

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] like any other data collator.

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 example below uses [DataCollatorMixin] because each training sample has a chosen and rejected response, and the model needs to see both.

Separate chosen_ids and rejected_ids because [~trl.trainer.utils.pad] expects flat lists.
Concatenate the input pair into a single list.
Generate attention_mask with torch.ones_like instead of the tokenizer because the collator works with raw token ID lists.
Pad input_ids and attention_mask.

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 for common tasks like token classification.