Flask

Warnings

• breaking @app.before_first_request and @bp.before_app_first_request decorators removed in Flask 3.0. Extremely common in older tutorials and LLM-generated code.
  Fix: Run initialization code in the app factory, in a CLI command, or using with app.app_context(): at startup. For teardown-based patterns use @app.teardown_appcontext.
• breaking FLASK_ENV environment variable, ENV config key, and app.env property removed in Flask 3.0. Using FLASK_ENV=development no longer enables debug mode.
  Fix: Use FLASK_DEBUG=1 or flask run --debug for debug mode. Set app.config['TESTING'] = True for test mode.
• breaking JSON config keys removed in Flask 3.0: JSON_AS_ASCII, JSON_SORT_KEYS, JSONIFY_MIMETYPE, JSONIFY_PRETTYPRINT_REGULAR. LLMs still generate these in config blocks.
  Fix: Use app.json.sort_keys = False, app.json.mimetype = 'application/json', app.json.ensure_ascii = False on the app.json provider instead.
• breaking app.json_encoder and app.json_decoder class attributes removed in Flask 3.0. Custom JSON serialization using these was a common pattern.
  Fix: Subclass flask.json.provider.DefaultJSONProvider and assign: app.json_provider_class = MyProvider, then app = Flask(__name__).
• breaking send_file() parameter names changed in Flask 2.0 and finalised in 3.0: attachment_filename → download_name, cache_timeout → max_age, add_etags → etag. Old names raise TypeError.
  Fix: Update send_file calls: send_file(path, download_name='file.csv', max_age=3600, etag=True).
• deprecated flask.Markup imported from flask is deprecated. Markup was moved to MarkupSafe.
  Fix: from markupsafe import Markup
• gotcha flask run requires the app to be discoverable. By default looks for app.py or wsgi.py with an app variable. Use FLASK_APP env var or --app flag for other module names.
  Fix: flask --app mymodule:app run or export FLASK_APP=mymodule:app
• gotcha Async views require Flask[async] (installs asgiref). Defining async def routes without this raises RuntimeError or silently runs synchronously depending on version.
  Fix: pip install 'Flask[async]' to enable async view support.

Install

• pip install Flask Core
• pip install "Flask[async]" With async view support (adds asgiref)
• pip install "Flask[dotenv]" With .env file support (adds python-dotenv)

Imports

• Flask

  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

  # 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

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