feat: default to bundled checkpoint

Fall back to the bundled uk_same model when no checkpoint is provided in the shared loader and fine-tune CLI. Keep tests aligned with the new default resolution behavior.

src/batdetect2/api_v2.py

@@ -656,7 +656,7 @@ class BatDetect2API:
     @classmethod
     def from_checkpoint(
         cls,
-        path: data.PathLike | str,
+        path: data.PathLike | str | None = None,
         audio_config: AudioConfig | None = None,
         train_config: TrainingConfig | None = None,
         evaluation_config: EvaluationConfig | None = None,

src/batdetect2/cli/finetune.py

@@ -5,6 +5,7 @@ import click
 from loguru import logger
 
 from batdetect2.cli.base import cli
+from batdetect2.train.checkpoints import DEFAULT_BUNDLED_CHECKPOINT
 
 __all__ = ["finetune_command"]
 
@@ -16,9 +17,12 @@ __all__ = ["finetune_command"]
 @click.option(
     "--model",
     "model_path",
-    required=True,
     type=str,
-    help="Path to a checkpoint or a Hugging Face URI to fine-tune from.",
+    help=(
+        "Path to a checkpoint, bundled checkpoint alias, or a Hugging Face "
+        "URI to fine-tune from. Defaults to "
+        f"'{DEFAULT_BUNDLED_CHECKPOINT}'."
+    ),
 )
 @click.option(
     "--targets",
@@ -106,7 +110,7 @@ __all__ = ["finetune_command"]
 )
 def finetune_command(
     train_dataset: Path,
-    model_path: str,
+    model_path: str | None,
     targets_config: Path,
     val_dataset: Path | None = None,
     ckpt_dir: Path | None = None,

src/batdetect2/train/checkpoints.py

@@ -8,19 +8,21 @@ from batdetect2.core import BaseConfig
 
 __all__ = [
     "CheckpointConfig",
+    "DEFAULT_CHECKPOINT",
     "build_checkpoint_callback",
     "get_bundled_checkpoint_names",
     "resolve_checkpoint_path",
 ]
 
+PACKAGE_ROOT = Path(__file__).resolve().parents[1]
 DEFAULT_CHECKPOINT_DIR: Path = Path("outputs") / "checkpoints"
-_PACKAGE_ROOT = Path(__file__).resolve().parents[1]
-_BUNDLED_CHECKPOINTS = {
-    "uk_same": _PACKAGE_ROOT
+DEFAULT_CHECKPOINT = "uk_same"
+CHECKPOINT_ALIASES = {
+    DEFAULT_CHECKPOINT: PACKAGE_ROOT
     / "models"
     / "checkpoints"
     / "batdetect2_uk_same.ckpt",
-    "batdetect2_uk_same": _PACKAGE_ROOT
+    "batdetect2_uk_same": PACKAGE_ROOT
     / "models"
     / "checkpoints"
     / "batdetect2_uk_same.ckpt",
@@ -32,7 +34,6 @@ class CheckpointConfig(BaseConfig):
     monitor: str | None = None
     mode: str = "max"
     save_top_k: int = 1
-    # Default to distributable inference checkpoints, not full train resumes.
     save_weights_only: bool = True
     filename: str | None = None
     save_last: bool | Literal["link"] = "link"
@@ -74,55 +75,55 @@ def build_checkpoint_callback(
 
 def get_bundled_checkpoint_names() -> tuple[str, ...]:
     """Return the supported bundled checkpoint aliases."""
-
-    return tuple(_BUNDLED_CHECKPOINTS)
+    return tuple(CHECKPOINT_ALIASES.keys())
 
 
-def resolve_checkpoint_path(path: PathLike | str) -> Path:
-    """Resolve a local path, bundled alias, or Hugging Face checkpoint URI.
+def resolve_checkpoint_from_huggingface(path: str) -> Path:
+    """Resolve a Hugging Face checkpoint URI."""
+    try:
+        from huggingface_hub import hf_hub_download
+    except ImportError as error:
+        raise ValueError(
+            "Hugging Face checkpoint support is not installed. "
+            "Install it with `pip install batdetect2[huggingface]`."
+        ) from error
+
+    repo_id, filename = _parse_huggingface_uri(path)
+    return Path(hf_hub_download(repo_id=repo_id, filename=filename))
+
+
+def resolve_checkpoint_path(path: PathLike | str | None = None) -> Path:
+    """Resolve a local path, alias, or Hugging Face checkpoint URI.
 
     Parameters
     ----------
-    path : PathLike | str
-        Local checkpoint path, bundled checkpoint alias, or a Hugging Face
-        URI of the form ``hf://owner/repo/path/to/checkpoint.ckpt``.
+    path : PathLike | str | None
+        Local checkpoint path, checkpoint alias, or a Hugging Face
+        URI of the form ``hf://owner/repo/path/to/checkpoint.ckpt``. If
+        omitted, the default alias checkpoint is used.
 
     Returns
     -------
     Path
         Resolved local filesystem path to the checkpoint.
     """
+    if path is None:
+        path = DEFAULT_CHECKPOINT
+
     if isinstance(path, str) and path.startswith("hf://"):
-        try:
-            from huggingface_hub import hf_hub_download
-        except ImportError as error:
-            raise ValueError(
-                "Hugging Face checkpoint support is not installed. "
-                "Install it with `uv sync --group huggingface` or "
-                "`pip install huggingface-hub`."
-            ) from error
+        return resolve_checkpoint_from_huggingface(path)
 
-        repo_id, filename = _parse_huggingface_uri(path)
-        return Path(hf_hub_download(repo_id=repo_id, filename=filename))
-
-    if not isinstance(path, Path):
-        path = Path(path)
+    if isinstance(path, str) and path in CHECKPOINT_ALIASES:
+        return Path(CHECKPOINT_ALIASES[path])
 
+    path = Path(path)
     if path.exists():
-        return path
-
-    bundled_path = _BUNDLED_CHECKPOINTS.get(str(path))
-    if bundled_path is not None:
-        if not bundled_path.exists():
-            raise FileNotFoundError(
-                f"Bundled checkpoint is missing: {bundled_path}"
-            )
-        return bundled_path
+        return path.resolve()
 
     bundled_names = ", ".join(get_bundled_checkpoint_names())
     raise FileNotFoundError(
-        "Checkpoint not found: "
-        f"{path}. Expected a local path, a bundled checkpoint alias "
+        f"Checkpoint not found: {path}. "
+        "Expected a local path, a checkpoint alias "
         f"({bundled_names}), or a Hugging Face URI."
     )
 

src/batdetect2/train/lightning.py

@@ -132,15 +132,15 @@ class StoredConfig:
 
 
 def load_model_from_checkpoint(
-    path: PathLike | str,
+    path: PathLike | str | None = None,
 ) -> tuple[Model, StoredConfig]:
     """Load a model and its configuration from a Lightning checkpoint.
 
     Parameters
     ----------
-    path : PathLike
+    path : PathLike | str | None
         Path to a ``.ckpt`` file produced by the BatDetect2 training
-        pipeline.
+        pipeline. If omitted, the default bundled checkpoint is used.
 
     Returns
     -------

tests/test_api_v2/test_api_v2.py

@@ -307,6 +307,12 @@ def test_checkpoint_with_same_targets_config_keeps_heads_unchanged(
         )
 
 
+def test_api_from_checkpoint_defaults_to_bundled_model() -> None:
+    api = BatDetect2API.from_checkpoint()
+
+    assert api.model.class_names
+
+
 @pytest.mark.slow
 def test_user_can_evaluate_small_dataset_and_get_metrics(
     api_v2: BatDetect2API,

tests/test_cli/test_finetune.py

@@ -1,6 +1,7 @@
 """CLI tests for finetune command."""
 
 from pathlib import Path
+from types import SimpleNamespace
 
 import pytest
 from click.testing import CliRunner
@@ -25,8 +26,41 @@ def test_cli_finetune_help() -> None:
     assert "--outputs-config" not in result.output
 
 
-def test_cli_finetune_requires_model() -> None:
-    """User story: finetune requires a checkpoint argument."""
+def test_cli_finetune_defaults_to_bundled_model(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """User story: finetune can use the bundled checkpoint by default."""
+
+    called = {}
+
+    class FakeAPI:
+        def finetune(self, **kwargs):
+            called["finetune"] = kwargs
+            return None
+
+    class FakeBatDetect2API:
+        @classmethod
+        def from_checkpoint(cls, path=None, **kwargs):
+            called["path"] = path
+            called["from_checkpoint_kwargs"] = kwargs
+            return FakeAPI()
+
+    monkeypatch.setattr(
+        "batdetect2.api_v2.BatDetect2API",
+        FakeBatDetect2API,
+    )
+    monkeypatch.setattr(
+        "batdetect2.data.load_dataset_config",
+        lambda path: SimpleNamespace(path=path),
+    )
+    monkeypatch.setattr(
+        "batdetect2.data.load_dataset",
+        lambda config, base_dir=None: [],
+    )
+    monkeypatch.setattr(
+        "batdetect2.targets.TargetConfig.load",
+        lambda path: SimpleNamespace(path=path),
+    )
 
     result = CliRunner().invoke(
         cli,
@@ -38,8 +72,9 @@ def test_cli_finetune_requires_model() -> None:
         ],
     )
 
-    assert result.exit_code != 0
-    assert "--model" in result.output
+    assert result.exit_code == 0
+    assert called["path"] is None
+    assert "finetune" in called
 
 
 def test_cli_finetune_requires_targets(tiny_checkpoint_path: Path) -> None:

tests/test_train/test_checkpoints.py

@@ -8,6 +8,7 @@ from soundevent import data
 
 from batdetect2.train import TrainingConfig, run_train
 from batdetect2.train.checkpoints import (
+    DEFAULT_BUNDLED_CHECKPOINT,
     get_bundled_checkpoint_names,
     resolve_checkpoint_path,
 )
@@ -144,13 +145,19 @@ def test_resolve_checkpoint_path_returns_local_path_unchanged(
 
 def test_get_bundled_checkpoint_names_lists_supported_aliases() -> None:
     assert get_bundled_checkpoint_names() == (
-        "uk_same",
+        DEFAULT_BUNDLED_CHECKPOINT,
         "batdetect2_uk_same",
     )
 
 
+def test_resolve_checkpoint_path_uses_default_bundled_alias() -> None:
+    resolved = resolve_checkpoint_path()
+
+    assert resolved == resolve_checkpoint_path(DEFAULT_BUNDLED_CHECKPOINT)
+
+
 def test_resolve_checkpoint_path_accepts_bundled_alias() -> None:
-    resolved = resolve_checkpoint_path("uk_same")
+    resolved = resolve_checkpoint_path(DEFAULT_BUNDLED_CHECKPOINT)
 
     assert resolved.name == "batdetect2_uk_same.ckpt"
     assert resolved.exists()