Flask

Flask#

#

Install#

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#

Key

Description

DEBUG

Enable debug mode

SECRET_KEY

For sessions/cookies

ENV

Set to ‘development’

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 %}
<p>Hi!</p>
{% 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
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