uvicorn-hmr

This package provides hot module reloading (HMR) for uvicorn.

It uses watchfiles to detect FS modifications, re-executes the corresponding modules with hmr and restart the server (in the same process).

HOT means the main process never restarts, and reloads are fine-grained (only the changed modules and their dependent modules are reloaded). Since the python module reloading is on-demand and the server is not restarted on every save, it is much faster than the built-in --reload option provided by uvicorn.

Why?

1. When you use uvicorn --reload, it restarts the whole process on every file change, but restarting the whole process is unnecessary:
   • There is no need to restart the Python interpreter, neither all the 3rd-party packages you imported.
   • Your changes usually affect only one single file, the rest of your application remains unchanged.
2. hmr tracks dependencies at runtime, remembers the relationships between your modules and only reruns necessary modules.
3. Since v0.7, hmr also tracks file system access - any file your code reads (config files, templates, data files, etc.) becomes reactive and will trigger reloads when modified.
4. So you can save a lot of time by not restarting the whole process on every file change. You can see a significant speedup for debugging large applications.
5. Although magic is involved, we thought and tested them very carefully, so everything works just as-wished.
   • Your lazy loading through module-level __getattr__ still works
   • Your runtime imports through importlib.import_module or even __import__ still work
   • Even valid circular imports between __init__.py and sibling modules still work
   • Fine-grained dependency tracking in the above cases still work
   • Decorators still work, even meta programming hacks like getsource calls work too
   • Standard dunder metadata like __name__, __doc__, __file__, __package__ are correctly set
   • ASGI lifecycles are preserved
6. We did some elegant engineering to optimize UX and performance of uvicorn:
   • Since the reloader and the server stay in the same thread, we can gracefully wait for the server to shutdown before reloading, which avoids long weird tracebacks that you will see when you keep pressing save during uvicorn --reload.
   • uvicorn's main loop has a 100ms tick interval, we optimized it into an event-driven model, so the server can respond to shutdown calls instantly!

Normally, you can replace uvicorn --reload with uvicorn-hmr and everything will work as expected, with a much faster refresh experience.

Installation

pip install uvicorn-hmr

pip install uvicorn-hmr[all]

Usage

Replace

uvicorn main:app --reload

with

uvicorn-hmr main:app

Everything will work as-expected, but with hot module reloading.

CLI Arguments

I haven't copied all the configurable options from uvicorn. But contributions are welcome!

For now, host, port, log-level, env-file are supported and have exactly the same semantics and types as in uvicorn.

The behavior of reload_include and reload_exclude is different from uvicorn in several ways:

1. Uvicorn allows specifying patterns (such as *.py), but in uvicorn-hmr only file or directory paths are allowed; patterns will be treated as literal paths.
2. Uvicorn supports watching non-Python files (such as templates), and uvicorn-hmr also supports hot-reloading when any accessed files are modified (including config files, templates, data files, etc.) through reactive file system tracking.
3. Uvicorn always includes/excludes all Python files by default (even if you specify reload-include or reload-exclude, all Python files are still watched/excluded accordingly), but uvicorn-hmr only includes/excludes the paths you specify. If you do not provide reload_include, the current directory is included by default; if you do provide it, only the specified paths are included. The same applies to reload_exclude.

The following options are supported but do not have any alternative in uvicorn:

• --refresh: Enables auto-refreshing of HTML pages in the browser whenever the server restarts. Useful for demo purposes and visual debugging. This is totally different from uvicorn's built-in --reload option, which is always enabled and can't be disabled in uvicorn-hmr because hot-reloading is the core feature of this package.
• --clear: Wipes the terminal before each reload. Just like vite does by default.

The two features above are opinionated and are disabled by default. They are just my personal practices. If you find them useful or want to suggest some other features, feel free to open an issue.