Properly connect PostgreSQL database to flask app with blueprints and without using SQLalchemy

I will do something like this:

The structure of the application will look like this:

├── application                    # hosts the application     
│   ├── __init__.py
│   ├── blueprint1                 
│   ├── blueprint2
├── application.py                 # contains the actual python code that will import the app
└── config.py       

Now inside your init.py, you can create the database connection like this

from flask import Flask
import psycopg2

db_connection = psycopg2.connect("dbname=suppliers user=postgres password=postgres")


def create_app(config_class=Config):
    application = Flask(__name__)
    application.config.from_object(config_class)

    # register blueprints here

    from application.blueprint1 import bp as blueprint1_bl

    application.register_blueprint(main_blueprint, url_prefix="/")

    from application.blueprint2 import bp as blueprint2_bl

    application.register_blueprint(
        blueprint2_bl, url_prefix="/bl2"
    )

    return application

And now let's say you have to access it from blueprint1 and blueprint1 is a separate directory. Then I will have a directory structure for each blueprint as below:

├── blueprint1 /
|   ├── __init__.py
|   └── static/blueprint1/                 
|   └── templates/blueprint1/                 
|   └── routes.py

Now from your routes.py file you can access the database connection as below:

from application import db_connection
from application.blueprint1 import bp

@bp.route("/", methods=["GET"])
def bl1_page():
    """It renders bl1 Page."""
    cursor = connection.cursor()
    postgreSQL_select_Query = "select * from mobile"

    cursor.execute(postgreSQL_select_Query)
    print("Selecting rows from mobile table using cursor.fetchall")
    mobile_records = cursor.fetchall()

    print("Print each row and it's columns values")
    for row in mobile_records:
        print("Id = ", row[0], )
        print("Model = ", row[1])
        print("Price  = ", row[2], "\n")
    return render_template(
        "blueprint1/blueprint1.html"
    )

And if you don't know how init.py file inside the blueprint will look like, it would look something like this

from flask import Blueprint

bp = Blueprint(
    "blueprint1",
    __name__,
    template_folder="templates",
    static_folder="static",
    static_url_path="/blueprint1/static",
)

from application.blueprint1 import routes

https://github.com/thefcraft/flask_postgresql

# pip install flask-pgsql --user

from flask_postgresql import PostgreSQL
db = PostgreSQL(hostname=hostname, port=port, database=database, username=username, password=password)

class Test(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    data = db.Column(db.Integer, array=True)

    def __repr__(self):
        return f"Test({self.id}, {self.data})" 

if __name__ == "__main__":
    if RESET:
        db.create_all() # with app.app_context(): db.create_all() will also work
    p = Test(data = [21, 24])
    db.session.add(p)
    db.session.commit()
    Test.query.get(id=1).data #-> [21, 24]