- SQLAlchemy (especially when using the
psycopg2 driver for PostgreSQL) requires the URI to be in the format postgresql+psycopg2://. - This prefix specifies the dialect (
postgresql) and the driver (psycopg2) explicitly, which SQLAlchemy uses to establish the connection.
Heroku's DATABASE_URL uses the older postgres:// scheme, which isn't compatible with the latest SQLAlchemy requirements. Starting with SQLAlchemy 1.4, postgres:// is considered deprecated and no longer supported. The explicit postgresql+psycopg2:// scheme is required.