Flask Basics

9 examples to get you started with Flask 3.1 - 6 basic and 3 intermediate.

Prerequisites

uv pip install "flask>=3.1"
export FLASK_APP=app:create_app
flask run

• Flask 3.1 on Python 3.14 uses Werkzeug 3.

Basic Examples

1. Minimal App

Hello world WSGI app.

from flask import Flask
 
app = Flask(__name__)
 
@app.get("/")
def home():
    return {"message": "Hello"}

• Flask(__name__) sets package root.
• Return dicts for JSON with Flask 3.
• Use flask run in development.

Related: Building REST APIs - JSON patterns

2. Route Methods

Restrict HTTP verbs.

from flask import Flask
app = Flask(__name__)
 
@app.route("/items", methods=["GET", "POST"])
def items():
    return {"ok": True}

• methods list controls verbs.
• 405 when verb not allowed.
• Prefer @app.get / @app.post shortcuts.

3. Query Args

Read query string parameters.

from flask import Flask, request
app = Flask(__name__)
 
@app.get("/search")
def search():
    q = request.args.get("q", "")
    return {"q": q}

• request.args is immutable mapping.
• Provide defaults for missing keys.
• Validate before passing to SQL.

4. JSON Body

Parse JSON POST bodies.

from flask import Flask, request
app = Flask(__name__)
 
@app.post("/items")
def create():
    data = request.get_json(force=True)
    return data, 201

• Check Content-Type in production.
• Validate with Pydantic or marshmallow.
• Return tuple for status code.

5. URL Variables

Dynamic path segments.

from flask import Flask
app = Flask(__name__)
 
@app.get("/users/<int:user_id>")
def user(user_id: int):
    return {"user_id": user_id}

• Converters coerce types.
• Type hints document intent.
• 404 via abort(404) when missing.

6. Error Handler

JSON errors.

from flask import Flask, abort
app = Flask(__name__)
 
@app.errorhandler(404)
def not_found(e):
    return {"error": "not_found"}, 404

• Register handlers per code.
• Keep payload shape stable.
• Log server-side details.

Related: Middleware patterns via hooks - lifecycle

Intermediate Examples

7. App Factory

Create app via factory function.

from flask import Flask
 
def create_app():
    app = Flask(__name__)
    app.config.from_mapping(DEBUG=True)
    return app

• Factories enable testing multiple configs.
• Register blueprints inside factory.
• Avoid module-level global app in libraries.

Related: Application Factories & Blueprints - blueprints

8. Blueprint

Modular routes.

from flask import Blueprint
bp = Blueprint("users", __name__, url_prefix="/users")
 
@bp.get("/")
def list_users():
    return []

• Blueprints group related routes.
• url_prefix shares path root.
• Register with app.register_blueprint.

9. Test Client

In-process HTTP tests.

def test_home(client):
    r = client.get("/")
    assert r.status_code == 200

• pytest fixture wraps app.test_client().
• No network required.
• Set TESTING config flag.

Related: Testing Flask - fixtures

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.