First Steps with JustAPI

This guide walks you through building and running your first JustAPI application from scratch.

from justapi import JustAPIApp

app = JustAPIApp(title="My First App", version="1.0.0")


@app.get("/")
def read_root(request):
    return {"status": "ok", "message": "Welcome to JustAPI!"}


@app.get("/users/{user_id}")
def read_user(request, user_id: int):
    return {"user_id": user_id, "active": True}

• JustAPIApp() initializes the Rust runtime — no separate server process needed
• @app.get("/") registers a route in the Rust radix-trie router (matchit)
• The request parameter is a zero-copy view into the HTTP request (no serialization overhead)
• Return a dict — JustAPI serializes it to JSON via serde_json in Rust

Run directly using Python:

python main.py

Or with uv:

uv run main.py

You should see output similar to:

[Rust Tokio Runtime] Listening on 127.0.0.1:8000
[Rust Tokio Runtime] Worker threads: 12
[Rust Tokio Runtime] OpenAPI docs at http://127.0.0.1:8000/docs

Or run using the hot-reloading dev server:

justapi serve --reload

Open a new terminal and test with curl:

curl http://127.0.0.1:8000/
# Output: {"status":"ok","message":"Welcome to JustAPI!"}

curl http://127.0.0.1:8000/users/42
# Output: {"user_id":42,"active":true}

curl http://127.0.0.1:8000/users/abc
# Output: {"detail":"validation error"}

Notice the third request returns a 422 validation error automatically — the Rust router rejected the non-integer user_id without ever calling Python.

JustAPI auto-generates OpenAPI 3.1 documentation. Open your browser to:

from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float
    is_offer: bool = None

@app.post("/items/")
def create_item(request, item: Item):
    return {"name": item.name, "price": item.price}

• Path & Query Parameters — Deep dive into routing
• Request Body & Validation — Schema validation with Pydantic
• CLI Project Scaffolder — Generate complete project templates