Skip to main content

Flask

Installation​

pip install Flask

Basic Application Structure​

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return "Hello, Flask!"

if __name__ == '__main__':
    app.run(debug=True)

Routing​

@app.route('/about')
def about():
    return "About Page"

@app.route('/user/<username>')
def show_user_profile(username):
    return f'User {username}'

@app.route('/post/<int:post_id>')
def show_post(post_id):
    return f'Post {post_id}'

HTTP Methods​

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        return 'Login POST request'
    else:
        return 'Login GET request'

Templates​

  • Setup

    • Create a folder named templates in your project directory.
    • Place HTML files in the templates folder.

  • Rendering Templates

from flask import render_template

@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
    return render_template('hello.html', name=name)
  • Template Example
templates/hello.html
<!doctype html>
<html>
<head><title>Hello</title></head>
<body>
  <h1>Hello, {{ name }}!</h1>
</body>
</html>

Static Files​

  • Setup:

    • Create a folder named static in your project directory.
    • Place static files (e.g., CSS, JavaScript, images) in the static folder.

  • Linking Static Files:

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

Forms and Request Data​

from flask import request

@app.route('/form-example', methods=['GET', 'POST'])
def form_example():
    if request.method == 'POST':
        name = request.form['name']
        return f'Hello, {name}!'
    return '''
        <form method="post">
            Name: <input type="text" name="name">
            <input type="submit">
        </form>
    '''

Redirects and Errors​

from flask import redirect, url_for, abort

@app.route('/redirect-example')
def redirect_example():
    return redirect(url_for('home'))

@app.route('/error-example')
def error_example():
    abort(404)

@app.errorhandler(404)
def page_not_found(error):
    return 'This page does not exist', 404

Sessions​

from flask import session

app.secret_key = 'supersecretkey'

@app.route('/set-session')
def set_session():
    session['username'] = 'admin'
    return 'Session set'

@app.route('/get-session')
def get_session():
    username = session.get('username')
    return f'Logged in as {username}'

Database (Flask-SQLAlchemy)​

  • Installation:
pip install Flask-SQLAlchemy
  • Setup:
from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)

    def __repr__(self):
        return f'<User {self.username}>'

db.create_all()
  • CRUD Operations:
# Create
new_user = User(username='admin', email='admin@example.com')
db.session.add(new_user)
db.session.commit()

# Read
user = User.query.filter_by(username='admin').first()

# Update
user.email = 'newemail@example.com'
db.session.commit()

# Delete
db.session.delete(user)
db.session.commit()

Running the Application​

export FLASK_APP=your_application.py
export FLASK_ENV=development
flask run

Flask Extensions​

  • Flask-WTF (Forms):
pip install Flask-WTF
  • Flask-Login (User Session Management):
pip install Flask-Login
  • Flask-Migrate (Database Migrations):
pip install Flask-Migrate

References​