Flask Skill

Production-tested patterns for Flask with the application factory pattern, Blueprints, and Flask-SQLAlchemy.

Latest Versions (verified January 2026):

• Flask: 3.1.2

• Flask-SQLAlchemy: 3.1.1

• Flask-Login: 0.6.3

• Flask-WTF: 1.2.2

• Werkzeug: 3.1.5

• Python: 3.9+ required (3.8 dropped in Flask 3.1.0)

Quick Start

Project Setup with uv

Create project

uv init my-flask-app cd my-flask-app

Add dependencies

uv add flask flask-sqlalchemy flask-login flask-wtf python-dotenv

Run development server

uv run flask --app app run --debug

Minimal Working Example

app.py

from flask import Flask

app = Flask(name)

@app.route("/") def hello(): return {"message": "Hello, World!"}

if name == "main": app.run(debug=True)

Run: uv run flask --app app run --debug

Known Issues Prevention

This skill prevents 9 documented issues:

Issue #1: stream_with_context Teardown Regression (Flask 3.1.2)

Error: KeyError in teardown functions when using stream_with_context

Source: GitHub Issue #5804 Why It Happens: Flask 3.1.2 introduced a regression where stream_with_context triggers teardown_request() calls multiple times before response generation completes. If teardown callbacks use g.pop(key) without a default, they fail on the second call.

Prevention:

WRONG - fails on second teardown call

@app.teardown_request def teardown_request(): g.pop("hello") # KeyError on second call

RIGHT - idempotent teardown

@app.teardown_request def teardown_request(): g.pop("hello", None) # Provide default value

Status: Will be fixed in Flask 3.2.0 as side effect of PR #5812. Until then, ensure all teardown callbacks are idempotent.

Issue #2: Async Views with Gevent Incompatibility

Error: RuntimeError when handling concurrent async requests with gevent Source: GitHub Issue #5881 Why It Happens: Asgiref fails when gevent monkey-patching is active. Asyncio expects a single event loop per OS thread, but gevent's monkey-patching makes threading.Thread create greenlets instead of real threads, causing both loops to run on the same physical thread and block each other.

Prevention: Choose either async (with asyncio/uvloop) OR gevent, not both. If you must use both:

import asyncio import gevent.monkey import gevent.selectors from flask import Flask

gevent.monkey.patch_all() loop = asyncio.EventLoop(gevent.selectors.DefaultSelector()) gevent.spawn(loop.run_forever)

class GeventFlask(Flask): def async_to_sync(self, func): def run(*args, **kwargs): coro = func(*args, **kwargs) future = asyncio.run_coroutine_threadsafe(coro, loop) return future.result() return run

app = GeventFlask(name)

Note: This "defeats the whole purpose of both" (maintainer comment). Individual async requests work, but concurrent requests fail without this workaround.

Issue #3: Test Client Session Not Updated on Redirect

Error: Session state incorrect after follow_redirects=True in tests Source: GitHub Issue #5786 Why It Happens: In Flask < 3.1.2, the test client's session wasn't correctly updated after following redirects.

Prevention:

If using Flask >= 3.1.2, follow_redirects works correctly

def test_login_redirect(client): response = client.post('/login', data={'email': 'test@example.com', 'password': 'pass'}, follow_redirects=True) assert 'user_id' in session # Works in 3.1.2+

For Flask < 3.1.2, make separate requests

response = client.post('/login', data={...}) assert response.status_code == 302 response = client.get(response.location) # Explicit redirect follow

Status: Fixed in Flask 3.1.2. Upgrade to latest version.

Issue #4: Application Context Lost in Threads (Community-sourced)

Error: RuntimeError: Working outside of application context in background threads Source: Sentry.io Guide Why It Happens: When passing current_app to a new thread, you must unwrap the proxy object using _get_current_object() and push app context in the thread.

Prevention:

from flask import current_app import threading

WRONG - current_app is a proxy, loses context in thread

def background_task(): app_name = current_app.name # Fails!

@app.route('/start') def start_task(): thread = threading.Thread(target=background_task) thread.start()

RIGHT - unwrap proxy and push context

def background_task(app): with app.app_context(): app_name = app.name # Works!

@app.route('/start') def start_task(): app = current_app._get_current_object() thread = threading.Thread(target=background_task, args=(app,)) thread.start()

Verified: Common pattern in production applications, documented in official Flask docs.

Issue #5: Flask-Login Session Protection Unexpected Logouts (Community-sourced)

Error: Users logged out unexpectedly when IP address changes Source: Flask-Login Docs Why It Happens: Flask-Login's "strong" session protection mode deletes the entire session if session identifiers (like IP address) change. This affects users on mobile networks or VPNs.

Prevention:

app/extensions.py

from flask_login import LoginManager

login_manager = LoginManager() login_manager.session_protection = "basic" # Default, less strict

login_manager.session_protection = "strong" # Strict, may logout on IP change

login_manager.session_protection = None # Disabled (not recommended)

Note: By default, Flask-Login allows concurrent sessions (same user on multiple browsers). To prevent this, implement custom session tracking.

Verified: Official Flask-Login documentation, multiple 2024 blog posts.

Issue #6: CSRF Protection Cache Interference (Community-sourced)

Error: Form submissions fail with "CSRF token missing/invalid" on cached pages Source: Flask-WTF Docs Why It Happens: If webserver cache policy caches pages longer than WTF_CSRF_TIME_LIMIT , browsers serve cached pages with expired CSRF tokens.

Prevention:

Option 1: Align cache duration with token lifetime

WTF_CSRF_TIME_LIMIT = None # Never expire (less secure)

Option 2: Exclude forms from cache

@app.after_request def add_cache_headers(response): if request.method == 'GET' and 'form' in request.endpoint: response.headers['Cache-Control'] = 'no-cache, no-store, must-revalidate' return response

Option 3: Configure webserver to not cache POST targets

In Nginx: add "proxy_cache_bypass $cookie_session" for form routes

Verified: Official Flask-WTF documentation warning, security best practices guides from 2024.

Issue #7: Per-Request max_content_length Override (New Feature)

Feature: Flask 3.1.0 added ability to customize Request.max_content_length per-request Source: Flask 3.1.0 Release Notes

Usage:

from flask import Flask, request

app = Flask(name) app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024 # 16MB default

@app.route('/upload', methods=['POST']) def upload(): # Override for this specific route request.max_content_length = 100 * 1024 * 1024 # 100MB for uploads file = request.files['file'] # ...

Note: Also added MAX_FORM_MEMORY_SIZE and MAX_FORM_PARTS config options in 3.1.0. See security documentation.

Issue #8: SECRET_KEY Rotation (New Feature)

Feature: Flask 3.1.0 added SECRET_KEY_FALLBACKS for key rotation Source: Flask 3.1.0 Release Notes

Usage:

config.py

class Config: SECRET_KEY = "new-secret-key-2024" SECRET_KEY_FALLBACKS = [ "old-secret-key-2023", "older-secret-key-2022" ]

Note: Extensions need explicit support for this feature. Flask-Login and Flask-WTF may need updates to use fallback keys.

Issue #9: Werkzeug 3.1+ Dependency Conflict

Error: flask==2.2.4 incompatible with werkzeug==3.1.3

Source: Flask 3.1.0 Release Notes | GitHub Issue #5652 Why It Happens: Flask 3.1.0 updated minimum dependency versions: Werkzeug >= 3.1, ItsDangerous >= 2.2, Blinker >= 1.9. Projects pinned to older versions will have conflicts.

Prevention:

Update all Pallets projects together

pip install flask>=3.1.0 werkzeug>=3.1.0 itsdangerous>=2.2.0 blinker>=1.9.0

Or with uv

uv add "flask>=3.1.0" "werkzeug>=3.1.0" "itsdangerous>=2.2.0" "blinker>=1.9.0"

Project Structure (Application Factory)

For maintainable applications, use the factory pattern with blueprints:

my-flask-app/ ├── pyproject.toml ├── config.py # Configuration classes ├── run.py # Entry point │ ├── app/ │ ├── init.py # Application factory (create_app) │ ├── extensions.py # Flask extensions (db, login_manager) │ ├── models.py # SQLAlchemy models │ │ │ ├── main/ # Main blueprint │ │ ├── init.py │ │ └── routes.py │ │ │ ├── auth/ # Auth blueprint │ │ ├── init.py │ │ ├── routes.py │ │ └── forms.py │ │ │ ├── templates/ │ │ ├── base.html │ │ ├── main/ │ │ └── auth/ │ │ │ └── static/ │ ├── css/ │ └── js/ │ └── tests/ ├── conftest.py └── test_main.py

Core Patterns

Application Factory

app/init.py

from flask import Flask from app.extensions import db, login_manager from config import Config

def create_app(config_class=Config): """Application factory function.""" app = Flask(name) app.config.from_object(config_class)

# Initialize extensions
db.init_app(app)
login_manager.init_app(app)

# Register blueprints
from app.main import bp as main_bp
from app.auth import bp as auth_bp

app.register_blueprint(main_bp)
app.register_blueprint(auth_bp, url_prefix="/auth")

# Create database tables
with app.app_context():
    db.create_all()

return app

Key Benefits:

• Multiple app instances with different configs (testing)

• Avoids circular imports

• Extensions initialized once, bound to app later

Extensions Module

app/extensions.py

from flask_sqlalchemy import SQLAlchemy from flask_login import LoginManager

db = SQLAlchemy() login_manager = LoginManager() login_manager.login_view = "auth.login" login_manager.login_message_category = "info"

Why separate file?: Prevents circular imports - models can import db without importing app .

Configuration

config.py

import os from dotenv import load_dotenv

load_dotenv()

class Config: """Base configuration.""" SECRET_KEY = os.environ.get("SECRET_KEY", "dev-secret-key") SQLALCHEMY_DATABASE_URI = os.environ.get("DATABASE_URL", "sqlite:///app.db") SQLALCHEMY_TRACK_MODIFICATIONS = False

class DevelopmentConfig(Config): """Development configuration.""" DEBUG = True

class TestingConfig(Config): """Testing configuration.""" TESTING = True SQLALCHEMY_DATABASE_URI = "sqlite:///:memory:" WTF_CSRF_ENABLED = False

class ProductionConfig(Config): """Production configuration.""" DEBUG = False

Entry Point

run.py

from app import create_app

app = create_app()

if name == "main": app.run()

Run: flask --app run run --debug

Blueprints

Creating a Blueprint

app/main/init.py

from flask import Blueprint

bp = Blueprint("main", name)

from app.main import routes # Import routes after bp is created!

app/main/routes.py

from flask import render_template, jsonify from app.main import bp

@bp.route("/") def index(): return render_template("main/index.html")

@bp.route("/api/health") def health(): return jsonify({"status": "ok"})

Blueprint with Templates

app/auth/init.py

from flask import Blueprint

bp = Blueprint( "auth", name, template_folder="templates", # Blueprint-specific templates static_folder="static", # Blueprint-specific static files )

from app.auth import routes

Database Models

app/models.py

from datetime import datetime from flask_login import UserMixin from werkzeug.security import generate_password_hash, check_password_hash from app.extensions import db, login_manager

class User(UserMixin, db.Model): """User model for authentication.""" tablename = "users"

id = db.Column(db.Integer, primary_key=True)
email = db.Column(db.String(120), unique=True, nullable=False, index=True)
password_hash = db.Column(db.String(256), nullable=False)
is_active = db.Column(db.Boolean, default=True)
created_at = db.Column(db.DateTime, default=datetime.utcnow)

def set_password(self, password):
    self.password_hash = generate_password_hash(password)

def check_password(self, password):
    return check_password_hash(self.password_hash, password)

def __repr__(self):
    return f"<User {self.email}>"

@login_manager.user_loader def load_user(user_id): return User.query.get(int(user_id))

Authentication with Flask-Login

Auth Forms

app/auth/forms.py

from flask_wtf import FlaskForm from wtforms import StringField, PasswordField, BooleanField, SubmitField from wtforms.validators import DataRequired, Email, Length, EqualTo, ValidationError from app.models import User

class LoginForm(FlaskForm): email = StringField("Email", validators=[DataRequired(), Email()]) password = PasswordField("Password", validators=[DataRequired()]) remember = BooleanField("Remember Me") submit = SubmitField("Login")

class RegistrationForm(FlaskForm): email = StringField("Email", validators=[DataRequired(), Email()]) password = PasswordField("Password", validators=[DataRequired(), Length(min=8)]) confirm = PasswordField("Confirm Password", validators=[ DataRequired(), EqualTo("password", message="Passwords must match") ]) submit = SubmitField("Register")

def validate_email(self, field):
    if User.query.filter_by(email=field.data).first():
        raise ValidationError("Email already registered.")

Auth Routes

app/auth/routes.py

from flask import render_template, redirect, url_for, flash, request from flask_login import login_user, logout_user, login_required, current_user from app.auth import bp from app.auth.forms import LoginForm, RegistrationForm from app.extensions import db from app.models import User

@bp.route("/register", methods=["GET", "POST"]) def register(): if current_user.is_authenticated: return redirect(url_for("main.index"))

form = RegistrationForm()
if form.validate_on_submit():
    user = User(email=form.email.data)
    user.set_password(form.password.data)
    db.session.add(user)
    db.session.commit()
    flash("Registration successful! Please log in.", "success")
    return redirect(url_for("auth.login"))

return render_template("auth/register.html", form=form)

@bp.route("/login", methods=["GET", "POST"]) def login(): if current_user.is_authenticated: return redirect(url_for("main.index"))

form = LoginForm()
if form.validate_on_submit():
    user = User.query.filter_by(email=form.email.data).first()
    if user and user.check_password(form.password.data):
        login_user(user, remember=form.remember.data)
        next_page = request.args.get("next")
        flash("Logged in successfully!", "success")
        return redirect(next_page or url_for("main.index"))
    flash("Invalid email or password.", "danger")

return render_template("auth/login.html", form=form)

@bp.route("/logout") @login_required def logout(): logout_user() flash("You have been logged out.", "info") return redirect(url_for("main.index"))

Protecting Routes

from flask_login import login_required, current_user

@bp.route("/dashboard") @login_required def dashboard(): return render_template("main/dashboard.html", user=current_user)

API Routes (JSON)

For REST APIs without templates:

app/api/init.py

from flask import Blueprint

bp = Blueprint("api", name)

from app.api import routes

app/api/routes.py

from flask import jsonify, request from flask_login import login_required, current_user from app.api import bp from app.extensions import db from app.models import User

@bp.route("/users", methods=["GET"]) @login_required def get_users(): users = User.query.all() return jsonify([ {"id": u.id, "email": u.email} for u in users ])

@bp.route("/users", methods=["POST"]) def create_user(): data = request.get_json() if not data or "email" not in data or "password" not in data: return jsonify({"error": "Missing required fields"}), 400

if User.query.filter_by(email=data["email"]).first():
    return jsonify({"error": "Email already exists"}), 409

user = User(email=data["email"])
user.set_password(data["password"])
db.session.add(user)
db.session.commit()

return jsonify({"id": user.id, "email": user.email}), 201

Register with prefix:

app.register_blueprint(api_bp, url_prefix="/api/v1")

Critical Rules

Always Do

• Use application factory pattern - Enables testing, avoids globals

• Put extensions in separate file - Prevents circular imports

• Import routes at bottom of blueprint init.py

• After bp is created

• Use current_app not app

• Inside request context

• Use with app.app_context()

• When accessing db outside requests

Never Do

• Never import app in models - Causes circular imports

• Never access db before app context - RuntimeError

• Never store secrets in code - Use environment variables

• Never use app.run() in production - Use Gunicorn

• Never skip CSRF protection - Keep Flask-WTF enabled

Common Errors & Fixes

Circular Import Error

Error: ImportError: cannot import name 'X' from partially initialized module

Cause: Models importing app, app importing models

Fix: Use extensions.py pattern:

WRONG - circular import

app/init.py

from app.models import User # models.py imports db from here!

RIGHT - deferred import

app/init.py

def create_app(): # ... setup ... from app.models import User # Import inside factory

Working Outside Application Context

Error: RuntimeError: Working outside of application context

Cause: Accessing current_app , g , or db outside request

Fix:

WRONG

from app import create_app app = create_app() users = User.query.all() # No context!

RIGHT

from app import create_app app = create_app() with app.app_context(): users = User.query.all() # Has context

Blueprint Not Found

Error: werkzeug.routing.BuildError: Could not build url for endpoint

Cause: Using wrong blueprint prefix in url_for()

Fix:

WRONG

url_for("login")

RIGHT - include blueprint name

url_for("auth.login")

CSRF Token Missing

Error: Bad Request: The CSRF token is missing

Cause: Form submission without CSRF token

Fix: Include token in templates:

<form method="post"> {{ form.hidden_tag() }} <!-- Adds CSRF token --> <!-- form fields --> </form>

Testing

tests/conftest.py

import pytest from app import create_app from app.extensions import db from config import TestingConfig

@pytest.fixture def app(): app = create_app(TestingConfig) with app.app_context(): db.create_all() yield app db.drop_all()

@pytest.fixture def client(app): return app.test_client()

@pytest.fixture def runner(app): return app.test_cli_runner()

tests/test_main.py

def test_index(client): response = client.get("/") assert response.status_code == 200

def test_register(client): response = client.post("/auth/register", data={ "email": "test@example.com", "password": "testpass123", "confirm": "testpass123", }, follow_redirects=True) assert response.status_code == 200

Run: uv run pytest

Deployment

Development

flask --app run run --debug

Production with Gunicorn

uv add gunicorn uv run gunicorn -w 4 -b 0.0.0.0:8000 "run:app"

Docker

FROM python:3.12-slim

WORKDIR /app COPY . .

RUN pip install uv && uv sync

EXPOSE 8000 CMD ["uv", "run", "gunicorn", "-w", "4", "-b", "0.0.0.0:8000", "run:app"]

Environment Variables (.env)

SECRET_KEY=your-production-secret-key DATABASE_URL=postgresql://user:pass@localhost/dbname FLASK_ENV=production

References

• Flask Documentation

• Flask-SQLAlchemy

• Flask-Login

• Flask Mega-Tutorial

• Application Factory Pattern

Last verified: 2026-01-21 | Skill version: 2.0.0 | Changes: Added 9 known issues (stream_with_context regression, async/gevent conflicts, test client sessions, threading context, Flask-Login session protection, CSRF cache, new 3.1.0 features, Werkzeug dependencies) Maintainer: Jezweb | jeremy@jezweb.net