robust-lid

Robust language identification that ensembles multiple LID backends into a single (language_script, confidence) prediction. Designed for short/noisy text where any single classifier is unreliable.

Install

pip install robust-lid                 # core ensemble (5 backends)
pip install 'robust-lid[cld3]'         # +CLD3 (requires system protoc)
pip install 'robust-lid[dev]'          # +dev tooling (ruff, mypy, pytest, …)

Quick start

Python

from robust_lid import RobustLID

lid = RobustLID()
code, confidence = lid.predict("The quick brown fox jumps over the lazy dog.")
# ('eng_Latn', 0.91)

CLI

Two entry points are registered: rlid (short) and robust-lid (long alias).

rlid "The quick brown fox jumps over the lazy dog."
# eng_Latn    0.987    The quick brown fox jumps over the lazy dog.

echo "안녕하세요" | rlid --json
# {"text": "안녕하세요", "lang": "kor_Hang", "confidence": 0.94}

rlid --file input.txt --no-text          # one pred per input line, no echo
rlid --models ft176,glotlid "Hello"      # use a subset of backends
rlid --uniform "Hello"                   # disable tuned defaults
rlid --low-memory "Hello"                # load one backend at a time (peak ~1.9 GB)
rlid --no-parallel "Hello"               # sequential predict (default is threaded)
rlid --verbose "Hello"                   # stage-by-stage progress on stderr
rlid --list-backends                     # inventory and exit
rlid --help

First call downloads ~1.5 GB of fastText models to ~/.cache/robust_lid/.

Batch prediction

For multi-text workloads, call predict_batch instead of predict in a loop:

from robust_lid import RobustLID

lid = RobustLID()
results = lid.predict_batch(["Hello world", "안녕하세요", "Bonjour"])
# [('eng_Latn', 0.99), ('kor_Hang', 1.0), ('fra_Latn', 0.97)]

The CLI automatically switches to the batch path when more than one text is provided (via --file or stdin).

Where the speedup actually comes from:

1. Single thread pool per batch instead of one per text — eliminates the 7-worker pool-construction overhead × N repetitions.
2. Cached label normalization in FastTextLID.predict_batch — the ISOConverter lookup runs once per distinct fasttext label across the batch, not k × N times.
3. fastText multilinePredict (C++) — a single C entry per backend. Measured benefit is small for already-fast models (lid.176: ~1× vs sequential) but more meaningful for the heavier fasttext-218e / GlotLID on large batches.

The unfixable bottleneck (and what fast_mode solves): per-backend wall time on N=200 English sentences, grouped by backend family:

langid and langdetect are pure-Python, GIL-bound, and have no batch API — they sit at ~95 % of total ensemble wall time regardless of how we call them.

fast_mode (default: on)

RobustLID(fast_mode=True) — which is the default — drops those two backends from the ensemble, leaving 4-5 all-C/C++ backends (cld2, cld3, ft176, ft218e, glotlid). This gives a large wall-time reduction with a small accuracy cost (fastText-176 alone already covers 176 languages — most of what langid+langdetect contribute).

from robust_lid import RobustLID
RobustLID()                   # fast_mode=True, 5-backend ensemble (default)
RobustLID(fast_mode=False)    # all 7 backends — maximum ensemble diversity

CLI equivalents:

rlid "text"                   # fast_mode default
rlid --with-slow "text"       # include langid + langdetect

SLOW_BACKEND_NAMES in robust_lid.ensemble exposes the excluded set (frozenset({"langid", "langdetect"})) for introspection.

Execution modes and memory footprint

Low-memory mode trades per-call latency for a much smaller resident footprint (useful on CI runners, small VPSes, or embedded-like environments). It disables supported-script gating — backends aren't live between calls, so their supported_scripts attribute can't be inspected. Incompatible with a custom models= list.

Backends

If gcld3 is not installed, the ensemble runs with 6 backends and emits an ImportWarning on package import.

Installation

pip install robust-lid

Optional: enable the CLD3 backend

gcld3 depends on the Protocol Buffers compiler (protoc), which must be installed at the system level before pip install:

Then:

pip install 'robust-lid[cld3]'

If you skip this, RobustLID will print:

ImportWarning: gcld3 is not installed; the CLD3 backend will be excluded from the RobustLID ensemble. ...

You can check availability at runtime:

from robust_lid.models import is_cld3_available
print(is_cld3_available())  # True or False

Development

uv sync --extra dev              # lint, mypy, pytest, pre-commit
uv sync --extra dev --extra e2e  # + datasets (WiLi-2018, papluca) + gcld3
uv run pytest                    # unit tests only (no network)
uv run pytest -m "slow and network"  # E2E — downloads ~1.5 GB of models on first run
uv run mypy src/robust_lid
uv run ruff check src/ tests/

LID benchmarks

The ensemble is evaluated on three Hub datasets with different domain characteristics and label granularity. Each dataset is compared at its native label granularity:

Why different comparison granularities? WiLi-2018 and papluca ship only language labels (no script), so we can't verify the script dimension against them. FLORES+ labels carry both, so matches_lang_script (in tests/integration/_common.py) enforces exact language AND exact script (modulo macrolanguage and script-supercode equivalence classes — so cmn_Hant ≡ zho_Hans and arb_Arab ≡ arz_Arab). This catches the class of bugs where a backend nails the language but mis-detects the script.

The 30 major languages tracked (tests/integration/_common.py) cover all commonly used scripts (Latn / Hang / Jpan / Hani / Hira / Kana / Arab / Cyrl / Deva / Beng / Thai / Grek / Hebr). Per-backend numbers are available via scripts/per_backend_accuracy.py --dataset wili papluca.

Running gated datasets (FLORES-200)

1. Copy the template: cp .env.example .env
2. Fill in a read-only Hugging Face token from https://huggingface.co/settings/tokens and accept the dataset's terms on its page.
3. Run: uv run pytest -m "slow and network" tests/integration/test_flores_e2e.py -s

.env is already in .gitignore. The token is loaded by tests/integration/conftest.py (stdlib-only parser — no extra dep). When HF_TOKEN is not set, FLORES tests skip automatically; the ungated benchmarks keep running.

Why project root? Every Python/Node tool (python-dotenv, pytest-env, Docker Compose, VS Code's python.envFile, Vercel/Netlify, etc.) searches for .env from the project root. A gitignored .env at root never shows up in git status, so the "clutter" cost is zero in practice.

Injecting fake backends (for tests)

from robust_lid import RobustLID
from robust_lid.models import LID

class FakeLID(LID):
    def predict(self, text: str) -> list[tuple[str, float]]:
        return [("eng", 0.99)]

lid = RobustLID(models=[FakeLID(), FakeLID()])  # no network, no models

FastTextLID(cache_dir=..., download_fn=..., model_loader=...) and ISOConverter(mapping=..., iso639_3_map=..., tsv_path=...) accept injected dependencies for testing.

Weighted voting

RobustLID combines three multiplicative knobs per backend:

Effective contribution of backend i:

weights[i] * script_weights[i].get(script, 1.0) * lang_weights[i].get(pred_lang, 1.0) * prob

Script-based backend gating

Each backend exposes supported_langs (ISO 639-3 codes it can ever emit) and supported_scripts (derived ISO 15924 codes). RobustLID uses the latter to auto-silence any backend whose supported-script set doesn't cover the input's detected script — preventing a backend from dragging the ensemble down with a confidently-wrong guess on text outside its coverage.

from robust_lid.models import LangdetectLID, FastText176LID

LangdetectLID().supported_langs       # frozenset of 55 ISO-639-3 codes
LangdetectLID().supported_scripts     # 41 ISO-15924 codes (Latn, Cyrl, …)
# langdetect has no Khmer, Ethiopic, Tibetan coverage;
# fastText-176 does, so on Amharic text only fastText's vote counts.

Custom backends (models=[MyLID(), ...]) default to frozenset() → gating is disabled. Override supported_langs to opt in.

RobustLID also applies two upstream-bug fixes at import time:

• langid returns raw log-probabilities by default (large negative values). LangidLID constructs the identifier with norm_probs=True so it yields [0, 1] probabilities; otherwise the negative vote totals flip sign during normalization and hand wins to whatever language langid disagreed with.
• fasttext-wheel 0.9.2 uses np.array(copy=False) which breaks on NumPy 2. _patch_fasttext_for_numpy2() monkey-patches _FastText.predict to use np.asarray instead.

compute_ensemble_vote also skips any vote with prob ≤ 0 defensively.

Calling RobustLID() without args auto-applies all three defaults. They were tuned on WiLi-2018 across 14 major languages to patch the known per-backend weak spots:

• langdetect × Hani → 0.3 (73 % recall on Chinese)
• langdetect × Jpan → 0.5
• cld2 × Hani → 0.8
• glotlid × Deva → 0.5 (confuses Hindi with Marathi)
• langid × {ltz, kir} → 0.5 / 0.3 (rare mis-labels of German/Turkish)
• glotlid × mar → 0.7
• ft176 scalar → 1.3, ft218e scalar → 1.2 (the two strongest backends)

Override or disable selectively:

from robust_lid import RobustLID
from robust_lid.ensemble import default_backend_order

order = default_backend_order()
# ['langid', 'langdetect', 'cld2', 'cld3', 'ft176', 'ft218e', 'glotlid']
# (cld3 is omitted if gcld3 isn't installed)

# Uniform (disable all tuning)
lid = RobustLID(
    weights=[1.0] * len(order),
    script_weights=[{}] * len(order),
    lang_weights=[{}] * len(order),
)

# Custom scalar weights by name
weights_by_name = {
    "langid": 1.0, "langdetect": 0.5, "cld2": 1.0, "cld3": 1.0,
    "ft176": 2.0, "ft218e": 2.0, "glotlid": 1.5,
}
lid = RobustLID(weights=[weights_by_name[name] for name in order])

For custom models (RobustLID(models=[...])) defaults are not applied because the tuning is keyed by backend name.

To measure per-backend accuracy on your own data and re-tune:

uv run python scripts/per_backend_accuracy.py --lang por deu tur --n 50

Release process

Releases are fully automated via python-semantic-release + PyPI Trusted Publishing. Every merge to main runs the release workflow, which:

1. Parses Conventional Commits since the last tag.
2. Decides the next semver (fix:/perf: → patch, feat: → minor, BREAKING CHANGE or type!: → major).
3. Updates pyproject.toml's project.version, appends to CHANGELOG.md, tags vX.Y.Z, and pushes a release commit.
4. Builds with uv build and uploads the wheel + sdist to PyPI via OIDC (no token secret — configured once as a PyPI trusted publisher).
5. Attaches the artifacts to the GitHub Release.

If no commit since the last release qualifies for a bump (only chore/docs/etc.), the workflow is a no-op.

Commit prefix cheat sheet

While major_on_zero = false (0.x lifecycle), breaking changes still bump minor rather than promoting to 1.0.