Flask cheatsheet

A concise cheatsheet for Flask, a lightweight WSGI web application framework for Python.

#Getting Started

#Install Flask

pip install flask

#Hello World

from flask import Flask
app = Flask(__name__)

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

app.run(debug=True)

#Flask CLI

export FLASK_APP=app.py
flask run

#App Config & Environment

#App Object

from flask import Flask
app = Flask(__name__)

#Configuration

app.config["DEBUG"] = True
app.config.from_pyfile("config.py")

#Common Config Keys

#Routing & Methods

#Basic Route

@app.route("/")
def home():
    return "Home"

#Methods

@app.route("/login", methods=["GET", "POST"])
def login():
    return "Login"

#Dynamic Routes

@app.route("/user/<name>")
def user(name):
    return f"User: {name}"

#Request & Response

#Request Object

from flask import request

@app.route("/data", methods=["POST"])
def data():
    return request.json

#Request Attributes

request.args: Query params
request.form: Form data
request.json: JSON body

#Custom Response

from flask import make_response

@app.route("/custom")
def custom():
    res = make_response("Custom", 200)
    res.headers["X-Test"] = "True"
    return res

#Templates & Static

#Render Template

from flask import render_template

@app.route("/")
def page():
    return render_template("index.html", name="Flask")

#Jinja Syntax

<h1>Hello {{ name }}</h1>
{% if name %}
Hi!
{% endif %}

#Static Files

<link href="{{ url_for('static', filename='style.css') }}" />

#Sessions & Cookies

#Using Sessions

from flask import session
app.secret_key = "secret"

@app.route("/login")
def login():
    session["user"] = "admin"
    return "Logged in"

#Access Cookies

from flask import request, make_response

@app.route("/cookie")
def cookie():
    resp = make_response("Set cookie")
    resp.set_cookie("site", "Flask")
    return resp

#JSON API & Middleware

#JSON Response

from flask import jsonify

@app.route("/api")
def api():
    return jsonify(message="Hello")

#Middleware

@app.before_request
def before():
    print("Before")

@app.after_request
def after(res):
    print("After")
    return res

#Error Handling & Blueprints

#Error Handling

@app.errorhandler(404)
def not_found(e):
    return "Page not found", 404

#Blueprint

from flask import Blueprint

admin = Blueprint("admin", __name__, url_prefix="/admin")

@admin.route("/")
def dash():
    return "Admin"

app.register_blueprint(admin)

#Project Structure

#File Structure

project/
├── app.py
├── templates/
│   └── index.html
├── static/
│   └── style.css
└── config.py

#Run Server

python app.py
# or using CLI
export FLASK_APP=app.py
flask run