larzflags

Feature flags with deterministic rollout & targeting. Pure Python, zero deps.

Ship code dark, turn it on for 5% of users, then 50%, then everyone - or enable it only for a plan, a country, or an allowlist. The key property is stable bucketing: a given user is always on the same side of a percentage rollout, so nobody flickers between on and off across requests.

from larzflags import FeatureFlags

flags = FeatureFlags()
flags.add("new_ui", enabled=True)
flags.add("beta", percentage=25)                       # 25% of users
flags.add("pro", rules=[("plan", "eq", "pro")])        # targeting
flags.add("vip", allow=["u1"], deny=["u2"])

flags.is_enabled("beta", user="alice")                 # stable per user
flags.is_enabled("pro", context={"plan": "pro"})       # True

Why

• Deterministic rollout. Percentage gating hashes flag + user, so a user's on/off state is stable across requests and processes - no random flicker, and ramping 10% -> 50% keeps the original 10% on.
• Targeting rules. ("attribute", op, value) with eq/ne/in/nin/gt/ lt/gte/lte/contains, ANDed together, against a context dict.
• Allow/deny lists override everything for specific users.
• A/B variants. variant() returns a weighted variant chosen deterministically per user - perfect for experiments.
• Zero dependencies. No SaaS, no SDK.

Install

pip install larzflags

Usage

from larzflags import FeatureFlags

flags = FeatureFlags()
flags.add("checkout_v2", percentage=10)
flags.add("eu_feature", rules=[("country", "in", ["DE", "FR", "ES"])])
flags.add("experiment", variants={"control": 50, "treatment": 50})

if flags.is_enabled("checkout_v2", user=user_id):
    ...
flags.variant("experiment", user=user_id)     # "control" or "treatment", stable
flags.all_enabled(user=user_id)               # set of on flags
flags.set_percentage("checkout_v2", 50)       # ramp up

Tests

python -m unittest discover -s tests -v   # 16 tests incl. rollout distribution

The Larz stack

One of 30+ pure-Python, zero-dependency libraries at github.com/larz-scripter.

License

MIT (c) larz-scripter