Flask stays minimal by design.

Extensions add features like:

• databases (Flask-SQLAlchemy)
• migrations (Flask-Migrate)
• login/session management (Flask-Login)
• forms (Flask-WTF)
• mail, caching, rate limiting, etc.

The extension pattern

Most extensions support this pattern:

# extensions.py
from flask_sqlalchemy import SQLAlchemy
from flask_login import LoginManager
 
 
db = SQLAlchemy()
login_manager = LoginManager()

# extensions.py
from flask_sqlalchemy import SQLAlchemy
from flask_login import LoginManager
 
 
db = SQLAlchemy()
login_manager = LoginManager()

Then inside your factory:

from .extensions import db, login_manager
 
db.init_app(app)
login_manager.init_app(app)

from .extensions import db, login_manager
 
db.init_app(app)
login_manager.init_app(app)

Why this matters

It avoids circular imports and allows:

• creating multiple app instances
• independent testing

Common extensions

• Flask-Login
• Flask-WTF
• Flask-Migrate
• Flask-Mail
• Flask-Admin
• Flask-Limiter (rate limiting)
• Flask-Caching

Pick extensions carefully and keep them consistent with your architecture.