What is SQLAlchemy?

SQLAlchemy is a Python SQL toolkit and Object-Relational Mapping (ORM) library. It provides a high-level interface for working with databases without writing raw SQL.

Installation

Install SQLAlchemy:

pip install sqlalchemy

Creating Engine

Connect to database:

from sqlalchemy import create_engine

# SQLite
engine = create_engine('sqlite:///database.db')

# PostgreSQL
engine = create_engine('postgresql://user:pass@localhost/dbname')

# MySQL
engine = create_engine('mysql://user:pass@localhost/dbname')

Defining Models

Create database models:

from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    
    id = Column(Integer, primary_key=True)
    username = Column(String(50), unique=True)
    email = Column(String(100), unique=True)

# Create tables
Base.metadata.create_all(engine)

Database Operations

Create Session

from sqlalchemy.orm import sessionmaker

Session = sessionmaker(bind=engine)
session = Session()

Create

user = User(username='alice', email='[email protected]')
session.add(user)
session.commit()

Read

# Get all
users = session.query(User).all()

# Get by ID
user = session.query(User).get(1)

# Filter
user = session.query(User).filter_by(username='alice').first()

Update

user = session.query(User).get(1)
user.email = '[email protected]'
session.commit()

Delete

user = session.query(User).get(1)
session.delete(user)
session.commit()

Best Practices

  • Use sessions properly: Close sessions after use
  • Handle transactions: Commit or rollback appropriately
  • Use migrations: Alembic for schema changes
  • Optimize queries: Use eager loading when needed

Next Steps

Learn about Flask database integration which uses SQLAlchemy.