Laco — Lazy Configuration for Reproducible ML

LAzy COnfiguration (Laco) is a Python-first configuration system that gives you type-checked, IDE-navigable configs with zero magic strings. It is compatible with Hydra and a drop-in alternative to hydra-zen.

Key ideas

30-second example

import laco
import laco.language as L
from torch import nn, optim

# --- Groups: swappable variants ---
class OptimGroup(L.Group[optim.Optimizer]):
    sgd  = L.partial(optim.SGD)(lr=L.required[float](), momentum=0.9)
    adam = L.partial(optim.Adam)(lr=1e-3)

# --- Schema: typed config ---
@L.config
class TrainSchema:
    epochs: int = 10
    optimizer: optim.Optimizer = L.slot(OptimGroup)

# --- Model ---
model_cfg = L.call(nn.Linear)(in_features=784, out_features=10)

# --- Task: runs with a composed config ---
@laco.main(config_name="train")
@L.task
def run(model: nn.Module, optimizer: optim.Optimizer, epochs: int = 10):
    opt = optimizer(model.parameters())
    # ... training loop ...

if __name__ == "__main__":
    run()

Override from the CLI:

python train.py optimizer=adam epochs=20
python train.py -m optimizer=sgd,adam optimizer.lr=1e-2,1e-3   # multirun

Tracing

@L.configurable
class Encoder:
    def __init__(self, depth: int) -> None: ...

# Records a config tree — no constructors run:
cfg = L.trace(lambda: Encoder(depth=24))
# Reproduce the object:
enc = laco.instantiate(cfg)

Examples

The sources/laco/examples/ directory contains a curriculum of examples:

Typed-group variants live in examples/typed/ (same models, @L.config + L.Group API).

CLI

laco compose configs/train.py            # load & dump YAML
laco compose configs/train.py lr=1e-3   # with overrides
laco show   configs/train.py --groups   # list registered groups
laco run    configs/train.py            # run the @L.task entry point
laco diff   configs/a.py configs/b.py   # diff two config files
laco app    my-project train --lr 1e-4  # run a laco.apps entry point

laco app — simple argparse CLI for published configs

Researchers and collaborators can reproduce experiments without knowing Hydra:

laco app my-project train --help          # show all available flags
laco app my-project train --lr 1e-4       # override a hyperparameter
laco app my-project train --dry-run       # preview resolved config as YAML
laco app my-project train --save-dir out/ # save config.yaml before running

Flags are derived from @L.params blocks and L.param() declarations in the config file. Authors register the entry point in pyproject.toml:

[project.entry-points."laco.apps"]
train = "my_project.train:run"

See docs/how-to/laco-app.md for the full author and user guide.

Migration from 0.x

Add import laco.compat once at startup to enable deprecation warnings for legacy _target_: laco.ops.partial configs. Run laco fix <dir> to rewrite them in-place.

Linting

Two lint passes run in CI and are available locally:

• nix run .#lint — ruff over sources/ and tests/.
• nix run .#lie-lint (or laco-lint sources/) — LACO001, a heuristic AST check for attribute access on lie-typed Laco nodes. Suppress with # noqa: LACO001.