abtestify

Deterministic A/B test bucketing for Python. Give it an experiment name and a user ID and it returns the same variant for that user every time, with no storage, no network calls and no dependencies.

Assignments are byte-for-byte identical to the abtestify npm package and its Go, Rust, Java/Kotlin and Swift ports, so a user lands in the same variant on every platform. Shared test vectors in the repository enforce it.

pip install abtestify

from abtestify import get_ab_experiment_bucket

assignment = get_ab_experiment_bucket(
    "checkout-cta",
    "user123",
    treatment_split=50,  # optional, default 50
    exposure=100,  # optional, default 100
)
if assignment.resolved_variant == "treatment":
    ...  # show the new checkout button

Keyword-only options: treatment_split, exposure, treatment_variant_name, control_variant_name, ignore_variant_name and legacy_hashing. The result is a frozen dataclass with experiment_name, unique_id and resolved_variant.

Invalid input raises TypeError (a non-numeric percentage or a non-string ID) or ValueError (a percentage that is not a finite number from 0 to 100, or an empty ID).

The bucket is the first 8 bytes of SHA-256 of experiment_name:unique_id, mapped to one of 10,000 buckets. Exposure takes the lowest buckets, and the split uses the bucket's last two digits, so raising exposure only adds users and raising treatment_split only moves users from control to treatment.

Requires Python 3.9 or later. Licensed under MIT.