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Different behavior with relative imports when using flask vs py
I have a small new Python 3.8 Flask project with some relative import quirks.
For the DB I use SQLAlchemy with Flask-Migrate. My project has the following general structure:
controllers/
static/
templates/
app.py
controller.py
helpers.py
models.py
requirements.txt
The app.py looks like this:
from .controller import app
if __name__ == "__main__":
app.run(host='0.0.0.0', port=5000, debug=True)
And the controller.py like this:
from flask import Flask
from . import helpers
app = Flask(__name__)
app.config.update(dict(
SQLALCHEMY_DATABASE_URI = "...",
SQLALCHEMY_TRACK_MODIFICATIONS = "..."
))
db = SQLAlchemy(app)
migrate = Migrate(app, db)
from .controllers.static_controller import static
app.register_blueprint(static, url_prefix='/')
The import in the model is this:
from .controller import db
And for the controllers the import is like this:
from flask import Blueprint, render_template, url_for, abort
from ..models import *
Using this setup I can run the application with the command flask run. However (likely due to a configuration error), I cannot use the restarter in that mode, because it shows an error that the python interpreter can't be found.
Hence I thought about using app.run (as you can see in the app.py), which does work in my experience. However, when I start the app with py app.py, I get this error:
Traceback (most recent call last):
File "app.py", line 2, in <module>
from .controller import app
ImportError: attempted relative import with no known parent package
I tried adding an __init__.py to the working directory, but that didn't help either.
My question is: How can I use relative imports here without drastically changing the project structure?
1 answer
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| luap42 | (no comment) | Oct 4, 2021 at 16:43 |
I don't have a py command on my system; for purposes of this answer I assume it's an alias to the python executable.
Running a script from a file is not quite the same as importing it. A script given on the command line doesn't know it's a package (__package__ is not set). That's why relative imports don't work.
What you probably want is to run it by module path, not filename. Assuming your top-level project directory is named yourapp:
python3 -m yourapp.app
Run this from somewhere that yourapp is importable. Unless you've done something odd with PYTHONPATH, the parent of your top-level project directory should work. You can check importability from the current directory with python3 -c 'import yourapp' or similar.
(even better would be to make a setup.py and use it to pip-install the package; then your working directory won't matter. But you specified no structural changes, so.)
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