Uvicorn Detect --reload Main Thread

Question

I have a simple Uvicorn application (built with FastAPI). In development, uvicorn server:app --reload. The --reload paramter means two threads will be started, one with the server, one to monitor file changes—when a file change is detected, the thread with the server is restarted.

Nothing special thus far. This is common in Flask, Django, and other dev servers as well. The catch is, I want to open a tunnel using pyngrok when I start the dev server, and I want that tunnel to be as long lived as the parent of these two threads. That is to say, when a file change is detected and the dev server is restarted, I do not want the tunnel to be restarted (since with ngrok this would cause a new public URL to be generated and break existing connections).

With Flask and Django this is relatively easy to accomplish. For example, with Flask I simply do os.environ.get("WERKZEUG_RUN_MAIN") != "true", with Django I similarly evaluate the RUN_MAIN environment variable, and in either case I only instantiate the tunnel when this these are set to true. I can't find a similar variable to use with Uvicorn. Can anyone help out here?

Answer 1

In FastAPI applications running with Uvicorn, you can check if the --reload option is enabled by examining the sys.argv list. The sys.argv list contains the command-line arguments passed to the script, and when the --reload option is used, it will be present in the list.

You can use the following code to determine if the --reload option is enabled:

import sys

def is_reload_enabled():
    return "--reload" in sys.argv

if is_reload_enabled():
    # Do not instantiate the tunnel when --reload is enabled
    print("Reload is enabled. Tunnel should not be started.")
else:
    # Instantiate the tunnel when --reload is not enabled
    print("Reload is disabled. Start the tunnel here.")