constants.py

from sqlalchemy import create_engine
from sqlalchemy.pool import NullPool
import os

# Create a SQLAlchemy connection string from the environment variable `DATABASE_URL`
# automatically created in your dash app when it is linked to a postgres container
# on Dash Enterprise. If you're running locally and `DATABASE_URL` is not defined,
# then this will fall back to a connection string for a local postgres instance
#  with username='postgres' and password='password'
connection_string = "postgresql" + os.environ.get(
    "DATABASE_URL", "postgresql+psycopg2://postgres:password@127.0.0.1:5432"
).lstrip("postgresql")

# Create a SQLAlchemy engine object. This object initiates a connection pool
# so we create it once here and import into app.py.
# `poolclass=NullPool` prevents the Engine from using any connection more than once. You'll find more info here:
# https://docs.sqlalchemy.org/en/14/core/pooling.html#using-connection-pools-with-multiprocessing-or-os-fork
postgres_engine = create_engine(connection_string, poolclass=NullPool,)