Flask

Common errors

Warnings

Install

Install compatibility verified last tested: 2026-05-12

Imports

• Flask
  wrong

  from flask import Flask
  app = Flask(__name__)
  app.config['FLASK_ENV'] = 'development'  # FLASK_ENV removed in 3.0

  correct

  from flask import Flask
  app = Flask(__name__)

  FLASK_ENV environment variable and app.env property were removed in Flask 3.0. Use FLASK_DEBUG=1 for debug mode instead.

• before_first_request
  wrong

  @app.before_first_request
  def initialize():
      init_db()  # decorator removed in Flask 3.0

  correct

  # Use app context or lifespan pattern
  with app.app_context():
      init_db()
  
  # Or with app factory + teardown:
  @app.before_request
  def check_initialized():
      ...

  @app.before_first_request and @bp.before_app_first_request removed in 3.0. Use app context at startup or initialize in app factory.

Quickstart verified last tested: 2026-04-23

Minimal Flask 3.x app with JSON responses and error handler.

from flask import Flask, jsonify, request

app = Flask(__name__)

@app.get('/')
def index():
    return jsonify({'status': 'ok'})

@app.post('/items')
def create_item():
    data = request.get_json()
    return jsonify(data), 201

@app.errorhandler(404)
def not_found(e):
    return jsonify({'error': 'not found'}), 404

# Run: flask --app app run --debug