spec-case (Python)

In-code spec / case / why / ideal / tmp / link / rule markers + specgen, the static extractor that compiles them into spec.json — the artifact ccr consumes. Part of spec-case; see the repo for concepts, the symbol-id contract, and the Go reference implementation.

Install

uv add spec-case        # or: pip install spec-case

One dependency covers both ends: the markers are imported by your code at runtime, and the same package ships the specgen console script for CI.

Use the markers

The markers are no-op decorators — at runtime each returns the function unchanged, so importing and annotating costs nothing and never changes behavior. They only mark functions for specgen's static (ast) extraction.

from spec_case import spec, case, why, ideal, tmp, link, rule

@spec("(tenant, name) unique; duplicate create -> ConflictError")
@case("happy_minimal", "only Name given should create", expect="201; body.id non-empty")
@case("duplicate_name", "duplicate Name", expect="409", forbid="a second row is written")
@why("database uniqueness is the cross-replica authority")
@ideal("one persistence owner replaces dual writes")
@tmp("dual-write during storage migration", until="all reads use the new store")
@link("component://docs/tenancy.md")
@rule("hot request path — watch new synchronous DB calls")
def create_notebook(req): ...

单个 spec 的 id 可省略。typing.overload 等同一 symbol 多声明场景可用 @spec("...", id="string_input") 区分；多个 spec 必须全部提供唯一 id。specgen 始终输出 统一的 {fqn?, specs[]} entry。

spec_case is zero-dependency (pure stdlib) and tiny, so taking it as a regular runtime dependency is cheap. Because the decorators apply at import time, the package must be importable anywhere the annotated module is imported (production included) — it is a runtime dependency, not dev-only.

Run specgen (CI)

Compile markers into spec.json, and gate drift in CI:

uv run specgen <src-dir> -o spec.json            # extract
uv run specgen <src-dir> -o spec.json --check    # CI gate: exit 1 if spec.json drifted

--check compares the committed spec.json against the current markers and fails if a symbol was renamed/removed or a marker changed. specgen parses with ast and never imports or runs your code.

until is optional: @tmp("keep fallback") records the measure without an exit condition.