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"""TRL-compatible `formatting_func` builder. |
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|
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Branches per row shape: |
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|
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- `"messages"` present → `tokenizer.apply_chat_template(msgs, tokenize=False)`. |
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SFTTrainer's completion-only loss masking kicks in automatically when |
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the formatted string is a chat transcript. |
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- `"text"` present → passthrough. SFTTrainer treats it as CPT (loss on |
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all tokens). |
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- neither → `DataFormatError`. |
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|
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PREFERENCE rows (`prompt`/`chosen`/`rejected`) are NOT formatted here — |
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they're routed to DPOTrainer, which has its own formatter. This |
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function refuses them explicitly so an accidentally-mixed dataset |
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fails loudly at format time rather than producing silently-wrong data. |
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""" |
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|
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from __future__ import annotations |
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|
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from collections.abc import Callable |
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from typing import TYPE_CHECKING, Any |
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|
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from dlm.data.errors import DataFormatError |
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|
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if TYPE_CHECKING: |
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from transformers import PreTrainedTokenizerBase |
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|
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Row = dict[str, Any] |
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FormattingFunc = Callable[[Row], str] |
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|
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|
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def make_formatting_func(tokenizer: PreTrainedTokenizerBase) -> FormattingFunc: |
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"""Return a row→str function bound to `tokenizer`'s chat template.""" |
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|
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def formatting_func(row: Row) -> str: |
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# HF `datasets.Dataset` unifies the schema across mixed-shape |
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# rows (e.g. PROSE + INSTRUCTION in one doc), filling missing |
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# columns with `None`. Dispatch on presence-and-non-None so a |
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# prose row with an injected `messages: None` doesn't route |
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# into `apply_chat_template` and crash Jinja with a None |
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# iterable. |
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messages = row.get("messages") |
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if messages is not None: |
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rendered = tokenizer.apply_chat_template( |
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messages, |
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tokenize=False, |
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add_generation_prompt=False, |
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) |
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if not isinstance(rendered, str): |
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raise DataFormatError( |
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f"apply_chat_template returned non-str ({type(rendered).__name__}); " |
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"ensure tokenize=False path is taken" |
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) |
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return rendered |
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text = row.get("text") |
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if text is not None: |
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if not isinstance(text, str): |
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raise DataFormatError(f"`text` field must be str, got {type(text).__name__}") |
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return text |
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if ( |
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row.get("prompt") is not None |
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and row.get("chosen") is not None |
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and row.get("rejected") is not None |
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): |
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raise DataFormatError( |
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"preference rows (prompt/chosen/rejected) must be routed to DPOTrainer, " |
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"not SFTTrainer's formatting_func" |
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) |
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raise DataFormatError(f"row has neither `messages` nor `text`: keys={sorted(row.keys())}") |
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|
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return formatting_func |