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Starlette middleware for Prerender.

Project description

Prerender Python Starlette

Starlette middleware for Prerender

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Google, Facebook, Twitter, and Bing are constantly trying to view your website... but Google is the only crawler that executes a meaningful amount of JavaScript and Google even admits that they can execute JavaScript weeks after actually crawling. Prerender allows you to serve the full HTML of your website back to Google and other crawlers so that they don't have to execute any JavaScript. Google recommends using to prevent indexation issues on sites with large amounts of JavaScript.

Prerender is perfect for Angular SEO, React SEO, Vue SEO, and any other JavaScript framework.

This middleware intercepts requests to your Node.js website from crawlers, and then makes a call to the (external) Prerender Service to get the static HTML instead of the JavaScript for that page. That HTML is then returned to the crawler.

README of prerender_rails

This library is a Python implementation of a Prerender middleware for Starlette. It should work flawlessly with FastAPI and, probably, with any ASGI framework.


pip install prerender-python-starlette


from starlette.applications import Starlette
from starlette.middleware import Middleware
from prerender_python_starlette import PrerenderMiddleware

routes = ...

middleware = [

app = Starlette(routes=routes, middleware=middleware)


  • prerender_service_url: URL of Prerender server. Defaults to PRERENDER_SERVICE_URL environment variable.
  • prerender_service_username: HTTP basic auth username of Prerender server. Defaults to PRERENDER_SERVICE_USERNAME environment variable.
  • prerender_service_password: HTTP basic auth password of Prerender server. Defaults to PRERENDER_SERVICE_PASSWORD environment variable.
  • prerender_service_token: Token set in X-Prerender-Token header. Defaults to PRERENDER_SERVICE_TOKEN environment variable.
  • crawler_user_agents: List of crawler user agents to intercept. Defaults to DEFAULT_CRAWLER_USER_AGENTS list.
  • extensions_to_ignore: List of file extensions to ignore. Defaults to DEFAULT_EXTENSIONS_TO_IGNORE list.
  • whitelist: List of path patterns to whitelist. Path not matching a pattern in the list won't be prerendered. Defaults to None.
  • blacklist: List of path patterns to blacklist. Path matching a pattern in the list won't be prerendered. Defaults to None.
  • before_render: Async function called before the prerendering. If it returns an HTMLResponse, it will be considered as cache and will bypass the call to the Prerender server. Defaults to None.
  • after_render: Async function called after the prerendering. Defaults to None.

Cache example

from starlette.applications import Starlette
from starlette.middleware import Middleware
from prerender_python_starlette import PrerenderMiddleware

async def before_render(request: Request) -> Optional[HTMLResponse]:
    cached_response = await cache.get(f"prerender:{request.url.path}")
    if cached_response:
        return HTMLResponse(cached_response)
    return None

async def after_render(
    request: Request, response: HTMLResponse, cached: bool
) -> None:
    if not cached:
        await cache.set(
            f"prerender:{request.url.path}", response.body.decode(response.charset)

routes = ...

middleware = [
  Middleware(PrerenderMiddleware, before_render=before_render, after_render=after_render),

app = Starlette(routes=routes, middleware=middleware)


Setup environement

You should have Pipenv installed. Then, you can install the dependencies with:

pipenv install --dev

After that, activate the virtual environment:

pipenv shell

Run unit tests

You can run all the tests with:

make test

Alternatively, you can run pytest yourself:


Format the code

Execute the following command to apply isort and black formatting:

make format


This project is licensed under the terms of the MIT license.

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