How to connect FastAPI to PostgreSQL using SQLAlchemy

Connecting FastAPI to PostgreSQL

SQLAlchemy is the standard ORM for Python. FastAPI has no built-in ORM — you wire SQLAlchemy manually. The convention is a database.py file that creates the engine and session factory.

Install dependencies

pip install sqlalchemy psycopg2-binary python-dotenv

database.py

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
import os

DATABASE_URL = os.getenv(
    "DATABASE_URL",
    "postgresql://user:password@localhost/dbname"
)

engine = create_engine(DATABASE_URL)

SessionLocal = sessionmaker(
    autocommit=False,
    autoflush=False,
    bind=engine
)

Base = declarative_base()

engine manages the connection pool. SessionLocal is a factory — call it to get a session object. Base is the base class all ORM models inherit from.

Inject the session with a yield dependency

from .database import SessionLocal

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

Never create sessions directly inside route handlers. Always inject via Depends(get_db) so sessions are guaranteed to close after each request, even on failure.