👀 Views: 36 💬 Answers: 1 📅 Created: 2025-06-28

python-3.x flask sqlalchemy database error-handling Python

I'm having a hard time understanding I'm testing a new approach and I tried several approaches but none seem to work... I'm working with a `sqlalchemy.exc.IntegrityError` when trying to commit a transaction in my Flask application using SQLAlchemy. The scenario seems to arise when I attempt to add a new user to the database. Here's the code snippet where the question occurs: ```python from flask import Flask, request, jsonify from flask_sqlalchemy import SQLAlchemy app = Flask(__name__) app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db' db = SQLAlchemy(app) class User(db.Model): id = db.Column(db.Integer, primary_key=True) username = db.Column(db.String(80), unique=True, nullable=False) @app.route('/add_user', methods=['POST']) def add_user(): username = request.json.get('username') new_user = User(username=username) db.session.add(new_user) try: db.session.commit() except Exception as e: db.session.rollback() return jsonify({'behavior': str(e)}), 400 return jsonify({'message': 'User added successfully!'}), 201 if __name__ == '__main__': db.create_all() app.run(debug=True) ``` When I send a POST request with a JSON payload to `/add_user`, I sometimes get the following behavior: `IntegrityError: UNIQUE constraint failed: user.username`. It seems like the behavior occurs only if I try to add a user with a username that already exists in the database, but I'm not always getting a clear indication of that before the attempt. I've checked if the user already exists by querying the database before adding a new user, but it seems that the transaction still goes through before it raises the behavior. Here's what I tried, which didn't resolve the scenario: ```python existing_user = User.query.filter_by(username=username).first() if existing_user: return jsonify({'behavior': 'User already exists!'}), 400 ``` Unfortunately, this check does not prevent the `IntegrityError` from happening if two requests are made simultaneously with the same username. How can I handle this situation properly in Flask with SQLAlchemy to avoid the integrity behavior and provide a user-friendly message? Any advice on best practices would be appreciated! This issue appeared after updating to Python 3.11. I'm working on a CLI tool that needs to handle this. I appreciate any insights!