Hello World in 2 Minutes

This tutorial walks through creating a complete JustAPI application, step by step. By the end, you’ll have a running API server with multiple endpoints.

Prerequisites

Python 3.11+
JustAPI installed (pip install justapi)

1. Create the App

Create a file named main.py:

from justapi import JustAPIApp

app = JustAPIApp(title="Hello World", version="0.1.0")


@app.get("/")
def root(request):
    return {"message": "Hello, World!"}

2. Run It

# Using Python directly
python main.py

# Or with UV (faster startup)
uv run main.py

Expected output:

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

3. Test It

curl http://127.0.0.1:8000/

Expected response:

{"message":"Hello, World!"}

4. Add More Endpoints

Extend your main.py with a parameterized route:

@app.get("/hello/{name}")
def hello_name(request, name: str):
    return {"greeting": f"Hello, {name}!"}

Test it:

curl http://127.0.0.1:8000/hello/JustAPI
# Output: {"greeting":"Hello, JustAPI!"}

5. Add a POST Endpoint

from pydantic import BaseModel

class Message(BaseModel):
    content: str
    author: str

@app.post("/messages/")
def create_message(request, msg: Message):
    return {
        "received": msg.content,
        "from": msg.author,
        "length": len(msg.content),
    }

Test with curl:

curl -X POST http://127.0.0.1:8000/messages/ \
  -H "Content-Type: application/json" \
  -d '{"content": "Hello from JustAPI!", "author": "Alice"}'

Expected response:

{"received":"Hello from JustAPI!","from":"Alice","length":20}

Complete Application

from justapi import JustAPIApp
from pydantic import BaseModel

app = JustAPIApp(title="Hello World", version="0.1.0")


class Message(BaseModel):
    content: str
    author: str


@app.get("/")
def root(request):
    return {"message": "Hello, World!"}


@app.get("/hello/{name}")
def hello_name(request, name: str):
    return {"greeting": f"Hello, {name}!"}


@app.post("/messages/")
def create_message(request, msg: Message):
    return {
        "received": msg.content,
        "from": msg.author,
        "length": len(msg.content),
    }


if __name__ == "__main__":
    app.run("127.0.0.1:8000")

Key Concepts

JustAPIApp() — Creates your application and initializes the Rust runtime
@app.get() / @app.post() — Route decorators that register handlers in the radix-trie router
Path parameters — {name} in the path becomes a function parameter
Pydantic models — Auto-validated request bodies with zero-copy parsing
app.run() — Starts the built-in Rust HTTP server (no uvicorn needed)

Next Steps

Path & Query Parameters — URL parameters, query strings, type validation
Request Body & Validation — Deep dive into schema validation
Dependency Injection — Reusable components with Depends