[FIXED] Flask app running with Flask but not Gunicorn

Issue

I created a Flask app for some of my projects. There is a main app.py in the ~/flasksvr directory, and the folder structure looks a bit like this:

flasksvr/
> __init__.py (empty)
> app.py (main app)
> modules/ (subprojects)
> > __init__.py (empty)
> > subproj1.py
> > subproj2.py
> > subproj3.py

When I run app.py, it imports all the subprojects with URL prefixes like this:

from .modules.subproj1 import app as sp1app
from .modules.subproj2 import app as sp2app
from .modules.subproj3 import app as sp3app

app.register_blueprint(sp1app, url_prefix='/sp1')
app.register_blueprint(sp2app, url_prefix='/sp2')
app.register_blueprint(sp3app, url_prefix='/sp3')

Anyways, running it with flask run -h 0.0.0.0 --debug works great, but when I run it with gunicorn app:app -b 0.0.0.0, it gives me the error

from .modules.subproj1 import app as sp1app
ImportError: attempted relative import with no known parent package

I tried changing the from .modules.subproj1 import app as sp1app part to from modules.subproj1 import app as sp1app, and it worked. But in my subproject 1, I am importing a SocketIO app with from ..app import sock (where sock is sock = SocketIO(app, ping_interval=10) in app.py). Then that gives me the error ImportError: Attempted relative import with no known parent package. Why is this happening? Why does it work with regular Flask but not gunicorn? Using Python 3.9 on Debian 11.

Solution

With folder structure

 flasksvr/
> __init__.py
> app.py         <----- path is here
> modules/
> > __init__.py  
> > subproj1.py  <----- in this file subproj1.py is `from app import sock` because your root path now at app.py
> > subproj2.py
> > subproj3.py

When you run project in gunicorn, Gunicorn will load all resource to worker and the root path is at app.py gunicorn app:app.It's different from running the flask run default, when call it will import

If you want to run both flask run and gunicorn. You can use:

flasksvr/
> app.py        <------ from modules.subproj1 import app as sp1app
> modules/
> > subproj1.py <------ from app import sock
> > subproj2.py
> > subproj3.py

Answered By - TanThien