lightcone

Strict-ordering bar feed for backtests and live trading strategies. Lookahead-proof by construction.

Only the past light cone influences the present.

Why this exists

Every major Python backtest framework has known lookahead pitfalls. The classic failure mode: each bar's "scene" / "context" object holds references to mutable state. Future bars mutate that state. By the time your trading loop reads bars[10].scene.zone.broken, the value reflects bars 11..N — your trading decision read the future.

Tests can catch known lookahead patterns. They cannot catch the unknown ones.

The structural approach: make it impossible for the strategy to see a bar before it has confirmed processing all earlier bars. Then lookahead bugs are not testable failures — they are architectural impossibilities.

That's lightcone.

How it works

1. Bars are delivered one at a time through a feed.
2. Each delivery returns an opaque token. The strategy MUST call confirm(token) before requesting the next bar.
3. Each bar is wrapped in a BarView that exposes only the fields the strategy declared in advance.
4. Backtest and live use the same API. Only the data source changes.

Install

pip install lightcone-feed

Zero dependencies. Python 3.9+.

Quick start

from lightcone import Lightcone, OHLCV, FeedExhausted, from_ohlcv_rows

# Build a feed from Binance/HL-style OHLCV rows
feed = from_ohlcv_rows(
    {("BTC", "5m"): btc_rows, ("ETH", "5m"): eth_rows},
    config=OHLCV,
)

while True:
    try:
        bar, key, token = feed.next_bar()
    except FeedExhausted:
        break

    # Your strategy reads only declared fields
    if bar.close > bar.open and bar.volume > some_threshold:
        decide_long(asset=key, ts=bar.ts)

    feed.confirm(token)   # ← MUST call before requesting next bar

Try to peek at a field you didn't declare → FieldNotDeclared. Try to skip confirm() and request the next bar → NotConfirmed. Try to confirm with the wrong token → BadToken.

Configuration profiles

from lightcone import CLOSE_ONLY, OHLCV, FULL_TAPE, custom

CLOSE_ONLY   # → ts + close. Discretionary / human-trader replay.
OHLCV        # → ts + OHLCV. Standard algorithmic backtest.
FULL_TAPE    # → OHLCV + n_trades + taker_buy. Microstructure.

cfg = custom("close", "volume", "n_trades")
cfg = custom("close", extras=["funding_rate"])

Multi-asset / multi-timeframe

feed = Lightcone(streams={
    ("BTC", "5m"):  btc_5m,
    ("BTC", "15m"): btc_15m,
    ("ETH", "5m"):  eth_5m,
})

Bars across all streams come out in strict timestamp order via a min-heap, with deterministic tie-breaking. Same API for one stream or fifty.

Fill simulation (for limit / stop strategies)

from lightcone import Order, Side, OrderType, simulate_fill

fill = simulate_fill(
    Order(Side.BUY, qty=1.0, order_type=OrderType.LIMIT, price=97.0),
    bar,
    slippage_bps=2.0,
)

The fill simulator reads the bar's full OHLC range — that's separate from the strategy's declared-field view, so your decision logic can't peek at high/low while the fill logic still uses them realistically.

Hardening

• Bars are frozen dataclasses; immutable post-construction
• Bar.extras is auto-frozen via MappingProxyType (external mutation can't leak in)
• BarView blocks all underscore-prefixed access — no view._bar.high escape hatch
• FieldNotDeclared does NOT inherit from AttributeError, so hasattr() cannot silently swallow it
• Tokens are 16 random bytes, compared with secrets.compare_digest
• Stream sequences are snapshotted to tuples — caller mutation can't affect the feed
• Strictly ascending timestamps required within each stream

Performance

The feed will never be your bottleneck. Strategy logic dominates by 10-100x.

What lightcone is NOT

• Not a full backtest framework. It's the data layer. Your strategy code, P&L tracking, and reporting live outside.
• Not thread-safe. Single-threaded use only. If parallelizing, give each thread its own feed.
• Not a training pipeline. Model training is offline batch work that uses the full timeline. Use lightcone for inference (backtest and live).
• Not a sandbox. A determined caller can read the feed's internal state. The contract catches accidents, not adversaries.

Documentation

Testing

pip install -e ".[dev]"
pytest lightcone/tests/

95 tests, ~14s runtime, 100% line coverage on production modules.

License

Apache-2.0. See LICENSE.

Contributing

See CONTRIBUTING.md. lightcone is small and focused — read that first.