arel

Browser hot reload for Python ASGI web apps.

Overview

What is this for?

arel can be used to implement development-only hot-reload for non-Python files that are not read from disk on each request. This may include HTML templates, GraphQL schemas, cached rendered Markdown content, etc.

How does it work?

arel watches changes over a set of files. When a file changes, arel notifies the browser (using WebSocket), and an injected client script triggers a page reload. You can register your own reload hooks for any extra server-side operations, such as reloading cached content or re-initializing other server-side resources.

Installation

pip install 'arel==0.5.*'

Quickstart

For a working example using Starlette, see the Example section.

Although the exact instructions to set up hot reload with arel depend on the specifics of your ASGI framework, there are three general steps to follow:

1. Create an HotReload instance, passing one or more directories of files to watch, and optionally a list of callbacks to call before a reload is triggered:

   import arel
   
   async def reload_data():
       print("Reloading server data...")
   
   hotreload = arel.HotReload(
       paths=[
           arel.Path("./server/data", on_reload=[reload_data]),
           arel.Path("./server/static"),
       ],
   )
   

2. Mount the hot reload endpoint, and wire its startup and shutdown into the application's lifespan. If using Starlette, this can be done like this:

   from contextlib import asynccontextmanager
   
   from starlette.applications import Starlette
   from starlette.routing import WebSocketRoute
   
   
   @asynccontextmanager
   async def lifespan(app):
       await hotreload.startup()
       try:
           yield
       finally:
           await hotreload.shutdown()
   
   
   app = Starlette(
       routes=[WebSocketRoute("/hot-reload", hotreload, name="hot-reload")],
       lifespan=lifespan,
   )
   

3. Add the JavaScript code to your website HTML. If using Starlette with Jinja templates, you can do this by updating the global environment, then injecting the script into your base template:

   templates.env.globals["DEBUG"] = os.getenv("DEBUG")  # Development flag.
   templates.env.globals["hotreload"] = hotreload
   

   <body>
     <!-- Page content... -->
   
     <!-- Hot reload script -->
     {% if DEBUG %}
       {{ hotreload.script(url_for('hot-reload')) | safe }}
     {% endif %}
   </body>
   

Example

The example directory contains an example Markdown-powered website that uses arel to refresh the browser when Markdown content or HTML templates change.

License

MIT

Changelog

All notable changes to this project will be documented in this file.

The format is based on Keep a Changelog.

0.5.0 - 2026-10-20

Added

• Add support for Starlette 1.x. This widens the starlette dependency to >=0.21,<2.0, and adds anyio>=3.4,<5 as a direct dependency. (Pull #42)
• Add optional nonce parameter to HotReload.script() (Pull #41)

Changed

• Users must now register hotreload.startup and hotreload.shutdown from a lifespan context manager. Starlette 1.0 removed the on_startup and on_shutdown arguments. Please see the updated README for usage. (Pull #42, #43)

0.4.0 - 2025-09-22

Removed

• Drop support for Python 3.7 and 3.8 (EOL versions). (Pull #39)

Added

• Add support for Python 3.13. (Pull #39)

Fixed

• Update watchfiles dependency to support modern versions (>=0.18,<2.0). (Pull #39)

0.3.0 - 2023-12-29

Changed

• Use watchfiles instead of watchgod. This unlocks Python 3.12 support. (Pull #34)

Added

• Add support for Python 3.12. (Pull #35)

0.2.0 - 2020-07-08

Added

• Add support for watching multiple directories, each with its own reload callbacks. (Pull #15)

Changed

• arel.HotReload("./directory", on_reload=[...]) should now be written as arel.HotReload(paths=[arel.Path("./directory", on_reload=[...])]). (Pull #15)

0.1.0 - 2020-04-11

Initial release.

Added

• Add HotReload ASGI application class. (Pull #1)