Python is where the rules become code.

Money is an int.

Minor units, always. No float ever touches a price, a fee, or a balance.

Unmeasured is None.

Not zero, not a mean, not a default. If a metric was not observed, the field says so and the caller has to handle it.

Typed, linted, tested, pinned.

Type hints throughout, ruff for lint and format, pytest with the reference's fake client so unit tests need no API key, and every third-party dependency pinned by commit or version.

Extend, never monkey-patch.

Upstream code is vendored read-only. Everything I add is a subclass or an extension, so an upstream update is a diff, not an archaeology dig.

# The shape of every tool in the harness.
def place_order(session: Session, draft_id: str) -> OrderResult:
    draft = session.stores.drafts.get(draft_id)      # server-issued id
    cap.check(session, draft.total_minor)            # cap on resulting state
    approval.require(session, "place_order", draft)  # human on the decision
    with session.serialized():                       # one write at a time
        order = session.stores.orders.commit(draft)
    audit.append(session, "place_order", order.id)   # chained record
    return OrderResult(order_id=order.id, total_minor=order.total_minor)