Flush the stdout buffer on Windows to trigger reload

[jamesh1999]

Summary

I'm raising this in response to #2000 as it's a fix I've been using locally. Through my experimentation, I have found that printing something after the Ctrl+C event is sent, allows the terminal running uvicorn to process the event, causing it to reload immediately.

I explain this in #2000 and there is already a reply saying it has resolved the issue for somebody else.

Tested on Windows 11 with:

• PyCharm (built-in terminal)
• VS Code (built-in terminal)
• Windows Terminal app

All of these now reliably reload for me. However I still have issues in CI running headlessly. I've started #2609 to discuss a fix for this too.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

Don't believe the last two points are necessary here but feel free to correct me.

[YousefBawaliz]

That seems to fix it. huge thanks man!
this issue has been bugging me for way too long now. Almost gave up FastAPI because of it.

let's hope it's merged as fast as possible.

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

uvicorn/supervisors/basereload.py:93

• Relying on a print statement may lead to unexpected output buffering behavior in some environments. Consider using sys.stdout.write("") followed by sys.stdout.flush() to ensure immediate output and to avoid side effects in production.

print("")  # This helps trigger windows to respond to the Ctrl+C event

[Kludex]

Can you try to use sys.stdout.flush() instead?

[jamesh1999]

So I did give this a go before and sys.stdout.flush() on its own doesn't seem to be enough. Based on Copilot's review, I also tried the following:

• sys.stdout.write("") then sys.stdout.flush() - No success here, it still doesn't trigger a reload.
• sys.stdout.write("\n") then sys.stdout.flush() - This is closer to print("") and does work. But at that point, I'm not sure it's any better than just using print("")

[jamesh1999]

Looks like sys.stdout.write(" ") then sys.stdout.flush() works too actually. That's a little less intrusive to the logs than printing a blank line and possibly a little bit clearer at showing what's required to get a reload to happen (something needs to be printed & the buffer need to be flushed)