FastAPI with on-demand HOT reload! 🙌

[CNSeniorious000]

Just like vite and vitest, hmr only reruns the changed file and affected modules. The auto-rerun is fine-grained and on-demand, so it is suitable for applications with rather long startup time.

hmr-demo.mov

Tip

You can try the example above here with hmr main.py

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About hmr, you can refer to its README for details.

[Kludex]

How does it handle or how would it handle static files if added to a web server?

[CNSeniorious000]

It don't affect any internal implementation of FastAPI / Starlette / uvicorn. So StaticFiles middleware works as before. hmr only reloads python modules.

Oh, let me clarify that hmr doesn't handle automatic refreshing on the frontend side. That's typically achieved through WebSockets or other long-polling technologies, which is a whole different story. What hmr actually does is just hot runtime reloading on the server-side.

[Kludex]

Let me rephrase it: I maintain Uvicorn. You pointed out on the README.md that it's a replacement to the --reload, so I'm asking how would hmr handle if added as one of the reload options to uvicorn in the case of static files.

[CNSeniorious000]

Ah I see. If I'm not mistaken, uvicorn doesn't directly handle static files; static files (which I understand to be non-Python files) are read from the disk by the ASGI application. However, since these are read using open, which I haven't patched yet, the functions that call open do not record a dependency on those static files. So, as you suspected, static files will not be hot-reloaded, which means overwriting a used static file won't trigger module reloads.

Of course, patching builtins.open to precisely record which static files a Python module depends on is possible. If you think this is necessary, I can try to create a demo.

However, I think this situation often arises in using template files. I've encountered this before as well. One approach is to use --reload-include *.j2 when starting uvicorn, and another is to disable the template engine's cache in the code, letting it to read from fs each time. I think the latter one is good enough for this usage.