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Basics

Installation​

pip install Flask

Basic Application Structure​

from flask import Flask

app = Flask(__name__)

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

@app.route('/user/<name>')
def user(name):
    return f'Hello, {name}!'

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

Running the Application​

Development Server​

# Set environment variables
export FLASK_APP=app.py
export FLASK_ENV=development

# Run the application
flask run

# Or run directly
python app.py

# Run on specific host and port
flask run --host=0.0.0.0 --port=5000

Application Factory Pattern​

from flask import Flask

def create_app(config_name='default'):
    app = Flask(__name__)
    app.config.from_object(config[config_name])

    from .main import main as main_blueprint
    app.register_blueprint(main_blueprint)

    return app

Routing​

Basic Routes​

@app.route('/')
def index():
    return 'Index Page'

@app.route('/hello')
def hello():
    return 'Hello World'

@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​

from flask import request

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

@app.route('/submit', methods=['POST'])
def submit():
    return 'Form submitted'

@app.route('/api/data', methods=['GET', 'POST', 'PUT', 'DELETE'])
def api_data():
    if request.method == 'GET':
        return get_data()
    elif request.method == 'POST':
        return create_data()
    elif request.method == 'PUT':
        return update_data()
    elif request.method == 'DELETE':
        return delete_data()

URL Variables​

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

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

@app.route('/path/<path:subpath>')
def show_subpath(subpath):
    return f'Subpath: {subpath}'

@app.route('/user/<uuid:user_id>')
def show_user_by_id(user_id):
    return f'User ID: {user_id}'

URL Building​

from flask import url_for

@app.route('/')
def index():
    return 'index'

@app.route('/login')
def login():
    return 'login'

@app.route('/user/<username>')
def profile(username):
    return f'{username}\'s profile'

with app.test_request_context():
    print(url_for('index'))  # /
    print(url_for('login'))  # /login
    print(url_for('profile', username='John'))  # /user/John

Configuration​

Configuration Variables​

class Config:
    SECRET_KEY = 'your-secret-key-here'
    DATABASE_URI = 'sqlite:///app.db'
    MAIL_SERVER = 'smtp.gmail.com'
    MAIL_PORT = 587
    MAIL_USE_TLS = True
    MAIL_USERNAME = 'your-email@gmail.com'
    MAIL_PASSWORD = 'your-password'

app.config.from_object(Config)

# Or load from environment
import os
app.config['SECRET_KEY'] = os.environ.get('SECRET_KEY')

Environment-based Configuration​

class Config:
    SECRET_KEY = os.environ.get('SECRET_KEY') or 'dev-secret-key'
    SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL') or 'sqlite:///app.db'
    SQLALCHEMY_TRACK_MODIFICATIONS = False

class DevelopmentConfig(Config):
    DEBUG = True
    SQLALCHEMY_DATABASE_URI = 'sqlite:///dev.db'

class ProductionConfig(Config):
    DEBUG = False
    SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL')

config = {
    'development': DevelopmentConfig,
    'production': ProductionConfig,
    'default': DevelopmentConfig
}

Request Context​

Request Object​

from flask import request

@app.route('/method')
def method():
    return request.method

@app.route('/headers')
def headers():
    return dict(request.headers)

@app.route('/user-agent')
def user_agent():
    return request.headers.get('User-Agent')

Application Context​

from flask import g, current_app

@app.before_request
def before_request():
    g.user = get_current_user()

@app.route('/user')
def user():
    return g.user.username if g.user else 'Anonymous'

Error Handling​

Error Handlers​

@app.errorhandler(404)
def not_found(error):
    return 'Page not found', 404

@app.errorhandler(500)
def internal_error(error):
    return 'Internal server error', 500

@app.errorhandler(Exception)
def handle_exception(e):
    return f'An error occurred: {str(e)}', 500

Custom Exceptions​

class ValidationError(Exception):
    pass

@app.errorhandler(ValidationError)
def handle_validation_error(e):
    return str(e), 400

@app.route('/validate')
def validate():
    if not request.args.get('data'):
        raise ValidationError('Data is required')
    return 'Valid data'

Logging​

Basic Logging​

import logging
from logging.handlers import RotatingFileHandler

if not app.debug:
    file_handler = RotatingFileHandler('logs/app.log', maxBytes=10240, backupCount=10)
    file_handler.setFormatter(logging.Formatter(
        '%(asctime)s %(levelname)s: %(message)s [in %(pathname)s:%(lineno)d]'))
    file_handler.setLevel(logging.INFO)
    app.logger.addHandler(file_handler)
    app.logger.setLevel(logging.INFO)
    app.logger.info('Application startup')

@app.route('/log')
def log_example():
    app.logger.info('Info message')
    app.logger.warning('Warning message')
    app.logger.error('Error message')
    return 'Check logs'

Static Files​

Serving Static Files​

# Static files go in 'static' folder
# CSS: static/css/style.css
# JS: static/js/app.js
# Images: static/images/logo.png

# In templates:
# <link rel="stylesheet" href="{{ url_for('static', filename='css/style.css') }}">
# <script src="{{ url_for('static', filename='js/app.js') }}"></script>
# <img src="{{ url_for('static', filename='images/logo.png') }}" alt="Logo">

Custom Static Folder​

app = Flask(__name__, static_folder='assets', static_url_path='/assets')

Testing​

Test Client​

import unittest

class FlaskTestCase(unittest.TestCase):
    def setUp(self):
        self.app = app.test_client()
        self.app.testing = True

    def test_index(self):
        result = self.app.get('/')
        self.assertEqual(result.status_code, 200)
        self.assertIn(b'Hello, World!', result.data)

    def test_login(self):
        result = self.app.post('/login', data={
            'username': 'test',
            'password': 'test'
        })
        self.assertEqual(result.status_code, 200)

if __name__ == '__main__':
    unittest.main()