A 'SQLAlchemy' instance has already been registered on this Flask app. Import and use that instance instead.

[Patrickcob]

If was curious about the way you made the unit tests in flask. I have a similar project with a very close architecture. I have a much more recent version of flask, flask-sqlalchemy and etc. I was curious if anyone was having problem with this project related to unit testing.

While creating an instance of the app in the test, it raise an error that RuntimeError: A 'SQLAlchemy' instance has already been registered on this Flask app. Import and use that instance instead.

From my understanding, when using flask test --coverage, it will create an instance of the app, than run the tests that try to create their own instances of the app. but in the create_app function, it tries to init the db with db.init_app(app) and it seems that, without surprise, there is a runtime error raised, because the database used is already in used, in this case, its a in-memory db. Something wrong or is this part of the newer flask and incompatible with this project?

[miguelgrinberg]

The problem is that you are trying to attach two Flask-SQLAlchemy extensions to the same Flask app instance. This suggests you are not creating a brand new app for each test, as is done in the tests in this project.

[saiteja-yadav]

myapp/init.py

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

Define db without initializing it yet

db = SQLAlchemy()

def create_app(test_config=None):
app = Flask(name)

# Load default config
app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///myapp.db"

# Override with test config if provided
if test_config:
    app.config.update(test_config)

# Initialize db with this app instance
db.init_app(app)

# Create tables (only if not testing, or handle in tests)
with app.app_context():
    db.create_all()

# Register routes, blueprints, etc.
from . import routes  # Assuming you have a routes module
app.register_blueprint(routes.bp)

return app

[saiteja-yadav]

Set Up Testing with Pytest
Use pytest fixtures to manage the app and database lifecycle. Create a conftest.py file in your test directory:

tests/conftest.py

import pytest
from myapp import create_app, db

@pytest.fixture(scope="session")
def app():
# Test config with in-memory SQLite
test_config = {
"SQLALCHEMY_DATABASE_URI": "sqlite:///:memory:",
"TESTING": True
}
app = create_app(test_config)
yield app

@pytest.fixture(scope="function")
def db_session(app):
# Set up a clean database for each test
with app.app_context():
db.create_all()
yield db
db.session.remove()
db.drop_all()

@pytest.fixture
def client(app):
# Test client for making HTTP requests
return app.test_client()