feat: support plain text files as prompt datasets (#337)

A dataset path that points to a plain file is now read as one prompt per
line, with empty lines ignored. For text files, "column" is ignored and
"split" is optional; when given, it selects a subset of lines using slice
notation (e.g. "[:400]").

Detection uses os.path.isfile so files without an extension also work. The
split-parsing logic is factored into a shared get_split_slice helper, which
derives the split name from the specification, and split/column are now
optional in DatasetSpecification, with the dataset branches raising a clear
error when either is missing. An invalid split raises instead of being
silently ignored.

A bare slice does not parse with the pinned datasets version, since
ReadInstruction.from_spec expects a named split, so the text branch prepends
a synthetic split name.

Revives the approach from #103.

Closes #98.

Co-authored-by: Ric <ricyoung@gmail.com>

config.default.toml

@@ -173,6 +173,11 @@ refusal_markers = [
 # System prompt to use when prompting the model.
 system_prompt = "You are a helpful assistant."
 
+# Each "dataset" below can be a Hugging Face dataset ID, a path to a dataset on disk,
+# or a path to a plain text file with one prompt per line (empty lines are ignored).
+# For text files, "column" is ignored and "split" is optional; when given, it selects
+# a subset of the lines using slice notation (e.g. "[:400]").
+
 # Dataset of prompts that tend to not result in refusals (used for calculating refusal directions).
 [good_prompts]
 dataset = "mlabonne/harmless_alpaca"

src/heretic/config.py

@@ -42,9 +42,15 @@ class DatasetSpecification(BaseModel):
         description="Hugging Face commit hash of the dataset.",
     )
 
-    split: str = Field(description="Portion of the dataset to use.")
+    split: str | None = Field(
+        default=None,
+        description="Portion of the dataset to use. Required for datasets, optional for plain text files.",
+    )
 
-    column: str = Field(description="Column in the dataset that contains the prompts.")
+    column: str | None = Field(
+        default=None,
+        description="Column in the dataset that contains the prompts. Required for datasets, ignored for plain text files.",
+    )
 
     prefix: str = Field(
         default="",

src/heretic/utils.py

@@ -188,6 +188,20 @@ class Prompt:
     user: str
 
 
+def get_split_slice(split_str: str, length: int) -> tuple[int, int]:
+    """Resolves a split specification into absolute (start, end) indices."""
+
+    # The split name is the part before the slice, e.g. "train" in "train[:400]".
+    split_name = split_str.split("[")[0]
+    # Associate the split with its number of examples (lines).
+    name_to_length = {split_name: length}
+    # Convert the instructions to absolute indices and select the first one.
+    absolute_instruction = ReadInstruction.from_spec(split_str).to_absolute(
+        name_to_length
+    )[0]
+    return absolute_instruction.from_, absolute_instruction.to
+
+
 def load_prompts(
     settings: Settings,
     specification: DatasetSpecification,
@@ -195,29 +209,41 @@ def load_prompts(
     path = specification.dataset
     split_str = specification.split
 
+    if os.path.isfile(path):
+        # Plain text file with one prompt per line. Empty lines are ignored.
+        with open(path, encoding="utf-8") as file:
+            prompts = [line.strip() for line in file if line.strip()]
+
+        # The split is optional for text files. When given, it selects a subset
+        # of the lines using slice notation (e.g. "[:400]"). A synthetic split
+        # name is prepended because ReadInstruction expects a named split.
+        if split_str is not None:
+            start, end = get_split_slice(f"_{split_str}", len(prompts))
+            prompts = prompts[start:end]
+    else:
+        # All dataset sources require an explicit split and column.
+        if split_str is None:
+            raise ValueError(f'The "split" field is required for datasets: {path}')
+
+        if specification.column is None:
+            raise ValueError(f'The "column" field is required for datasets: {path}')
+
         if is_hf_path(path):
             dataset = load_dataset(
                 path,
                 revision=specification.commit,
                 split=split_str,
             )
-    else:
-        if Path(path, DATASET_STATE_JSON_FILENAME).exists():
+        elif Path(path, DATASET_STATE_JSON_FILENAME).exists():
             # Dataset saved with datasets.save_to_disk; needs special handling.
             # Path should be the subdirectory for a particular split.
             dataset = load_from_disk(path)
             assert not isinstance(dataset, DatasetDict), (
                 "Loading dataset dicts is not supported"
             )
-            # Parse the split instructions.
-            instruction = ReadInstruction.from_spec(split_str)
-            # Associate the split with its number of examples (lines).
-            split_name = str(dataset.split)
-            name2len = {split_name: len(dataset)}
-            # Convert the instructions to absolute indices and select the first one.
-            abs_instruction = instruction.to_absolute(name2len)[0]
-            # Get the dataset by applying the indices.
-            dataset = dataset[abs_instruction.from_ : abs_instruction.to]
+            # Parse the split instructions and apply them.
+            start, end = get_split_slice(split_str, len(dataset))
+            dataset = dataset[start:end]
         else:
             # Path should be a local directory.
             dataset = load_dataset(