Run uvicorn programmatically with `reload=True` not in if-name-main block print irrelated error message

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

When you run uvicorn programmatically, if __name__ == '__main__' block is required if you put reload=True as parameter to uvicorn.run function. Or you will got

INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [$PID] using watchgod
ERROR:    [Errno 48] Address already in use

I checked all tcp ports in LISTEN status, 8000 port is free to use and all processes I can recognize them.(They are not zombie watchgod process or something like that).

Using command like uvicorn --reload app:app works perfectly in same project.

Put entry code into if-name-main block solved the problem.

Sorry of my poor English, I’m not a native speaker.😅

To reproduce

• app.py

import fastapi
app = fastapi.FastAPI()

• dev.py

import uvicorn
uvicorn.run('app:app', reload=True)

run python dev.py will print error message said Address already in use.

Expected behavior

Best: Put entry code into if-name-main block is not required.

Good: Print error message or other information to indicate the real problem.

Write warning or note in doc.

Actual behavior

Print irrelated error message Address already in use.

Debugging material

INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [$PID] using watchgod
ERROR:    [Errno 48] Address already in use

Environment

• OS / Python / Uvicorn version: MacOS 11.3.1 / Python 3.9.5 / uvicorn 0.13.4

[euri10]

if I do python dev.py I have this error which is quite explicit about multiprocessing, since we dont use fork but spawn (see https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods)

RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

so indeed you have to use the __main__ block if you want to run uvicorn However I’m unable to reproduce your Address already in use message, probably a difference in os, can someone on a mac see if he can ?

[euri10]

I think this issue is likely caused by same reason with #949. IMO, write something in doc to point the proper idiom in the main module is required while using some options like ‘debug/reload’ is not a bad thing anyway.

ok let’s close those 2 issues by adding a warning to the docs, fancy sending a PR @TLDay ?