python 47 lines · 7 steps

Per-request SQLAlchemy sessions in Flask

A request-scoped database session that opens lazily and commits or rolls back automatically when each request ends.

Explained by highlit
Flask Intermediate database sessions request lifecycle connection management transactions dependency injection
1from flask import Flask, g, jsonify, request, abort
2from sqlalchemy import create_engine
3from sqlalchemy.orm import scoped_session, sessionmaker
4 
5app = Flask(__name__)
6engine = create_engine(app.config.get("DATABASE_URL", "postgresql://localhost/app"), future=True)
7SessionFactory = sessionmaker(bind=engine, autoflush=False, expire_on_commit=False)
8 
9 
10def get_session():
11 if "db_session" not in g:
12 g.db_session = SessionFactory()
13 return g.db_session
14 
15 
16@app.teardown_appcontext
17def close_session(exc):
18 session = g.pop("db_session", None)
19 if session is None:
20 return
21 if exc is None:
22 session.commit()
23 else:
24 session.rollback()
25 session.close()
26 
27 
28@app.post("/users")
29def create_user():
30 payload = request.get_json(silent=True) or {}
31 email = payload.get("email")
32 if not email:
33 abort(422, description="email is required")
34 
35 session = get_session()
36 user = User(email=email, name=payload.get("name"))
37 session.add(user)
38 session.flush()
39 return jsonify(id=user.id, email=user.email), 201
40 
41 
42@app.get("/users/<int:user_id>")
43def show_user(user_id):
44 user = get_session().get(User, user_id)
45 if user is None:
46 abort(404)
47 return jsonify(id=user.id, email=user.email, name=user.name)
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Tying a session to Flask's request-scoped `g` gives each request its own isolated database connection without global state.
  2. 2Committing or rolling back in `teardown_appcontext` centralizes transaction handling so individual routes never manage it manually.
  3. 3Lazy creation via a helper means a session is only opened when a route actually touches the database.

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