Jar 🫙

Jar is a toy Python web framework, implemented in about 200 lines of code (see cli.py). It's open source.

I built it to explore some ideas around framework APIs. Please don't actually use it.

It deploys to Vercel via the Build Output API.

It's called Jar because it has almost no features and you need to fill it up yourself!

Features

Jar uses file-system routing.

Pages are Python files that render content. They're put in the pages directory.

• Build pages aka Static Files
• Fresh pages aka Serverless Functions
• Regenerated pages aka Prerender Functions

Public files (like CSS and other media files) go in the public directory and are served from the root path.

A typical project is structured like this:

project/
├─ pages/
│  ├─ index.py
├─ public/
│  ├─ favicon.ico

Checkout the source for the kitchen sink example, or this docs website.

Build pages

Generated once at build time. Served as a static file.

import time

def render(data):
    return f"<h1>{data['text']} I was built at {data['time']}</h1>", {}

def data():
    return {
        "text": "Hello, World!",
        "time": time.time(),
    }

Fresh pages

Generated for each request. Similar to Server-Side Rendering (SSR).

import json
import time


def render(data):
    content = f"<h1>Fresh Page rendered at {data['time']}</h1>"
    content += f"<code>{data['request']}</code>"
    return content, {}


def data(request):
    return {
        "time": time.time(),
        "request": json.dumps({
            "method": request.method,
            "path": request.path,
            "headers": request.headers,
            "body": request.body
        }, indent=4)
    }


def config():
    return {
        "fresh": {}
    }

Regenerated Pages

Similar to Next.js's Incremental Static Regeneration.

import time


def render(data):
    return f"<h1>Regenerated Page, last rendered at {data['time']}</h1>", {}

def data(request=None):
    return {
        "time": time.time(),
    }

def config():
    return {
        "regenerate": {
            "every": 5
        }
    }

More on the API

The data and config functions are optional. The properties that config returns defines the type of page. The default page is a build page.

Build pages don't have access to a request object.

The response that render returns is a tuple of body, meta where meta can have status_code: int and/or headers: dict keys e.g. {"status_code": 200, "headers": {"some":"header"}}.

More on Packages

If you are using packages (i.e. something you install with pip) you have to install them locally inside your project before building the project with the CLI.

e.g. with pip you can run pip3 install -r requirements.txt --target . at the project's root.

See how this docs website is deployed for an example of this (deploy-docs.sh).