uvicorn-hmr

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 reload 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?

• 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.
• hmr tracks dependencies at runtime, remembers the relationships between your modules and only reruns necessary modules.
• 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.
• 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

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

Installation

pip install uvicorn-hmr

Or with extra dependencies:

pip install uvicorn-hmr[all]

This will install fastapi-reloader too, which enables you to use --reload flag to reload the browser pages when the server restarts.

When you enable the --reload flag, it means you want to use the fastapi-reloader package to enable automatic HTML page reloading. This behavior differs from Uvicorn's built-in --reload functionality. (See the configuration section for more details.)

Server reloading is a core feature of uvicorn-hmr and is always active, regardless of whether the --reload flag is set. The --reload flag specifically controls auto-reloading of HTML pages, a feature not available in Uvicorn.

If you don't need HTML page auto-reloading, simply omit the --reload flag. If you do want this feature, ensure that fastapi-reloader is installed by running: pip install fastapi-reloader or 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:

• 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.
• Uvicorn supports watching non-Python files (such as templates), but uvicorn-hmr currently only supports hot-reloading Python source files.
• 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:

• --reload: 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.