Flask

library3.1.3pypypi✓ verified 25d ago

Lightweight WSGI web framework. Current version is 3.1.3 (Feb 2026). Flask 3.0 (Sep 2023) removed a large set of APIs that had been deprecated since 2.x — before_first_request, FLASK_ENV, JSON config keys, and custom json_encoder/decoder. LLMs still generate these removed patterns.

web-framework serialization

pip install Flask

github ↗package ↗homepage ↗docs ↗changelog ↗

INSTALL

IMPORT

SIG · FLASK

Flask

web-frameworkpythonv3.1.3

Install

2.6s avg

Import

461ms

Disk

21MB

Pass rate

10/ 10

Env Coverage10 / 10

glibc

3.9–3.13

musl

3.9–3.13

Install & Compatibility

Where this runs

tested against v3.1.3 · pip install

no network on importno background threads

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

py 3.10–3.915 runs

installs and imports cleanly · install 0.0s · import 0.483s · 23MB

glibc

py 3.10–3.915 runs

installs and imports cleanly · install 2.6s · import 0.439s · 23MB

21MB installed

● package 21MB

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

Flask

✓ from flask import Flask app = Flask(__name__)

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

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 def initialize(): init_db() # decorator removed in Flask 3.0

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

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

flask --version

Debug

Known issues

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.

affects: >= 3.0

breakingFLASK_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.

affects: >= 3.0

breakingJSON 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.

affects: >= 3.0

breakingapp.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__).

affects: >= 3.0

breakingsend_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).

affects: >= 2.0

deprecatedflask.Markup imported from flask is deprecated. Markup was moved to MarkupSafe.

fix

from markupsafe import Markup

affects: >= 2.0

gotchaflask 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

affects: all

gotchaAsync 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.

affects: >= 2.0

Errors

Common errors & fixes

ImportError: cannot import name 'FlaskForm' from 'flask'

The 'FlaskForm' class is part of the 'flask_wtf' extension, not the core 'flask' module.

fix

Use 'from flask_wtf import FlaskForm' instead of 'from flask import FlaskForm'.

ImportError: cannot import name 'url_quote' from 'werkzeug.urls'

The 'url_quote' function was removed in Werkzeug 3.0.0.

fix

Downgrade Werkzeug to a version before 3.0.0 by adding 'Werkzeug==2.3.7' to your 'requirements.txt' file and reinstalling dependencies.

ImportError: cannot import name 'escape' from 'jinja2'

The 'escape' function was removed from Jinja2 in version 3.1.0.

fix

Import 'escape' from 'markupsafe' instead: 'from markupsafe import escape'.

ImportError: cannot import name 'Flask' from 'flask'

This error often occurs due to a circular import or incorrect project structure.

fix

Ensure that your project structure does not have circular imports and that 'flask' is installed correctly.

ImportError: cannot import name 'json' from 'flask'

The 'json' module was removed from Flask in version 3.0.0.

fix

Use Python's built-in 'json' module instead: 'import json'.

Upgrade

Version history

3.1.3latest on PyPI · released Feb 19, 2026

Audit

Dependencies

Werkzeug>=3.1requiredWSGI utilities. Required. Version pinned by Flask.

Jinja2>=3.1.2requiredTemplate engine. Required.

asgiref>=3.2optionalRequired for async views. Only installed with Flask[async].

python-dotenvoptionalLoads .env files for flask run. Only installed with Flask[dotenv].

Agent activity

133 hits · last 30 days

node

119

Anthropic

OpenAI (training)

Resources

githubhttps://github.com/pallets/flask ↗

packagehttps://pypi.org/project/Flask/ ↗

homepagehttps://flask.palletsprojects.com ↗

docshttps://flask.palletsprojects.com/ ↗

changeloghttps://flask.palletsprojects.com/page/changes/ ↗