jiti 1.3.3

Just In Time Implementation: declare a typed function or method by its interface and let an LLM write, validate, and cache a real implementation.

jiti

Interface-first Python. Declare the interfaces, wire them into a call graph, run the program — an LLM writes the implementations the first time each one is called, and the result is real, committable code that you keep.

The idea

You decide what: typed signatures, docstrings, and tests. You decide how the pieces fit: which function calls which. Then you run, and bodies appear just in time, get validated against ruff + ty + your tests, and land as plain Python under .jiti/. Every call after that is plain dispatch — no model, no API key, no network. When you're ready, jiti merge folds the generated code back into your source and removes the decorator.

You can stop using jiti at any time and keep everything it wrote.

A tiny example

from jiti import jiti


@jiti
def slugify(text: str) -> str:
    """Convert text to a URL-safe slug."""
    ...

The first call to slugify("Hello, World!") runs an agent that inspects the real arguments, explores your repo, drafts code, runs ruff + ty + any tests you've gated on it, and writes the result to a file beside your source. Every call after that runs that file.

Wiring a graph

Interface-first pays off when interfaces compose. You write the orchestration in plain Python — that's your code, and that's where the graph lives. jiti writes the leaves.

from jiti import jiti


@jiti
def satisfies(version: str, spec: str) -> bool:
    """True if `version` satisfies `spec`. Specs: exact, '>=', '>', '<=', '<', '~', '^'."""
    ...


@jiti
def sort_versions(versions: list[str]) -> list[str]:
    """Return the version strings sorted ascending by semver precedence."""
    ...


# Your code — plain Python — composing the jiti pieces:
def latest_matching(versions: list[str], spec: str) -> str | None:
    """Return the highest-precedence version satisfying `spec`, or None."""
    candidates = [v for v in versions if satisfies(v, spec)]
    return sort_versions(candidates)[-1] if candidates else None

latest_matching is yours — no decorator, no magic, just a function. The first call to latest_matching(["1.0.0", "2.0.0", "2.1.3"], "^2.0.0") runs your code, which calls satisfies and sort_versions, which jiti generates on demand (and which may themselves need other stubs along the way — generation cascades). Every call after that is plain dispatch.

The full runnable version (with a Version dataclass, more stubs, and a method) lives in examples/semver/.

Install

pip install jiti     # or: uv add jiti

Needs Python 3.13+. Generation uses LiteLLM and defaults to Claude Sonnet 4.6; set ANTHROPIC_API_KEY for the default model, or set JITI_MODEL to any LiteLLM model id and provide that provider's API key. Running already-generated code needs nothing — no key, no network.

ANTHROPIC_API_KEY=... python your_script.py
OPENAI_API_KEY=... JITI_MODEL=openai/<model-id> python your_script.py

Stubs

A stub is a function with a docstring and a placeholder body: ..., pass, or raise NotImplementedError. A real body is an error — @jiti means "write this for me." A comment in the stub becomes a hint:

@jiti
def parse_money(raw: str) -> Decimal:
    """Parse a currency string like '$1,234.56' into a Decimal."""
    # strip the symbols and separators, then Decimal()
    ...

Methods and async def functions work the same way — decorate the targets you want generated, use self freely on methods, and await async targets normally (see Version.bump in examples/semver/core.py).

Strict type checkers flag an empty body with a non-None return (empty-body). That's your checker reacting to the stub, not jiti. Disable that rule or use raise NotImplementedError.

Test-driven generation

State a function's definition of done from your test file with @jiti.required_for(target). Tests import the real code, so the reference is type-checked — and running pytest is the loop: generation happens to make your tests pass, red → green.

# tests/test_money.py
from app.money import parse_money
from jiti import jiti


@jiti.required_for(parse_money)        # real body → your gate test, run as-is
def test_parses_symbols():
    assert parse_money("$1,234.56") == Decimal("1234.56")


@jiti.required_for(parse_money)        # empty body → jiti writes this test from the interface
def test_rejects_garbage() -> None:
    """parse_money raises ValueError on '' and 'not money'."""
    ...

An empty-bodied stub is a jiti-test: written before the implementation exists, so it can only see the interface and can't couple to internals. jiti writes it, commits it under .jiti/tests/, and gates the implementation on it. Both are ordinary test_* functions your own pytest run executes.

Graduating off jiti

Interface-first is a development mode you can leave. jiti merge folds the generated implementation back into your source, replaces the stub, removes @jiti, and cleans up the mirror:

jiti status                  # what's generated, what you've hand-edited
jiti merge app.text.slugify  # inline one function into its source
jiti merge --all             # graduate the entire project

After merge --all, you have plain Python, no jiti dependency required. See docs/reference.md for the full CLI and configuration surface.

A few things worth knowing

• The code is yours. Edit a generated body and jiti runs it as-is — it tracks a hash and won't clobber your edits. Change a stub's signature, docstring, or gates and it regenerates; if you'd hand-edited that section, it surfaces a conflict instead.
• git is yours. jiti only writes files into .jiti/. Commit it (so production runs cached code with no key) or gitignore it. jiti never runs git.
• Concurrency. Running generated code is fully safe — it's plain dispatch. Generating does no locking, so warm the cache once single-threaded, then parallelize. Writes are atomic, so a reader never sees a half-written file.

Where to go next

• examples/semver/ — a runnable interface-first walkthrough: stubs, a graph, tests, and the merge graduation.
• docs/reference.md — every Engine knob, env var, and CLI flag.
• CHANGELOG.md — release history.

Development

uv sync
uv run pre-commit install

The gate, exactly what CI runs — ruff-format, ruff, ty, then pytest (no API key; uses a fake client):

uv run pre-commit run --all-files
uv run pytest

Status

Today, jiti supports sync and async free functions and methods, lazy agentic generation with cascading across the call graph, in-process validation (ruff + ty + pytest), test-driven generation via @jiti.required_for (works on free functions and methods), a score-gated refactor pass, common decorator stacks (@staticmethod, @classmethod, @property, and functools wrappers), the edit/conflict lifecycle, and the jiti CLI (status / merge / test / clear). Anthropic, OpenAI, Gemini, and other model families are available through LiteLLM.

Not yet: a pytest plugin, whole-class generation, generation locking, and dependency-aware invalidation.

Inspiration

This video is what inspired me to make this package. I then found this package, a joke implementation of the idea from 2 years ago. But now we've gotten to the point where LLMs are good enough that maybe, just maybe, this direction is worth entertaining.