Sqlalchemy.exc.nosuchmoduleerror: Can't load plugin: sqlalchemy.dialects:postgres

Updating to 'postgresql' schema

SQLAlchemy 1.4+ strictly requires using postgresql:// prefix in your database URI. It's not asking for much:

from sqlalchemy import create_engine

# You said 'postgres', I say 'postgresql'
engine = create_engine('postgresql://username:password@localhost/dbname')
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Deploying an application using Flask? Substitute SQLALCHEMY_DATABASE_URI to the postgresql:// protocol:

# 'postgres' is not in vogue anymore!
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://username:password@localhost/dbname'
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If you're a Heroku fan, remember to update your configuration variables to display postgresql://.

AppConfig madness

For configuration files like alembic.ini, ensure the sqlalchemy.url key conforms to the postgresql:// protocol:

[alembic]
# Giving SQLAlchemy what it wants:
sqlalchemy.url = postgresql://username:password@localhost/dbname
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Prefer to specify the driver explicitly? Use "postgresql+psycopg2://". SQLAlchemy doesn’t mind:

# Driver explicit, just like my playlist 
engine = create_engine('postgresql+psycopg2://username:password@localhost/dbname')
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Regardless of your workflow, ensure your database URL's prefix matches SQLAlchemy's expectations.

Automate your way out

If you need to programmatically rectify DATABASE_URL within your application, Python's built-in libraries like os and re are more than happy to help:

import os
import re

# Calling Mr. Environment Variable
DATABASE_URL = os.getenv('DATABASE_URL')

fixed_url = re.sub(r'^postgres://', 'postgresql://', DATABASE_URL)

# Now creating a drama-free engine
engine = create_engine(fixed_url)
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This code snippet arms you with an automated method of keeping your URIs SQLAlchemy-compliant.

Solving common quirks

Even with the correct URI, issues might crop up. But don't fear, solutions are here:

Check for typos

Ensure no spaces or unwanted characters have snuck into your connection string.

Confirm psycopg2 installation

Make sure psycopg2-binary or psycopg2 resides in your virtual environment and both are updated.

URL's Format

Ensure your URL always sticks to postgresql://user:pass@localhost/dbname. A slight deviation might create big headaches.