# Flask Quick Reference *Routes, templates, requests, blueprints, database, extensions* > Source: Flask Documentation (flask.palletsprojects.com) · MIT ## Setup ### Minimal App ``` from flask import Flask app = Flask(__name__) @app.route('/') def index(): return 'Hello, World!' ``` ### Run the App ``` pip install flask flask --app app run --debug # or: python -m flask run --debug ``` ### Project Structure | Command | Description | |---------|-------------| | `app.py` | Application entry point | | `templates/` | Jinja2 HTML templates | | `static/` | CSS, JS, images | | `models.py` | Database models | | `requirements.txt` | Python dependencies | ## Routes ### Basic Routes ``` @app.route('/about') def about(): return render_template('about.html') @app.route('/user/') def profile(username): return f'User: {username}' ``` ### URL Variables | Command | Description | |---------|-------------| | `` | String (default) | | `` | Integer | | `` | Float | | `` | String with slashes | | `` | UUID | ### HTTP Methods ``` @app.route('/login', methods=['GET', 'POST']) def login(): if request.method == 'POST': return do_login() return render_template('login.html') ``` ### URL Building ``` from flask import url_for url_for('profile', username='alice') # => '/user/alice' ``` ## Templates ### Render Template ``` from flask import render_template @app.route('/posts') def posts(): items = get_posts() return render_template('posts.html', posts=items) ``` ### Jinja2 Syntax ``` {{ variable }} {% if user %}Welcome, {{ user.name }}!{% endif %} {% for item in items %}

{% endfor %} ``` ### Template Inheritance ``` {# base.html #} {% block content %}{% endblock %} {# child.html #} {% extends "base.html" %} {% block content %}

Page

{% endblock %} ``` ### Common Filters | Command | Description | |---------|-------------| | `\|safe` | Render raw HTML | | `\|escape` | HTML-escape string | | `\|length` | Count items | | `\|default('N/A')` | Fallback for empty | | `\|tojson` | Serialize to JSON | ## Request & Response ### Request Object ``` from flask import request request.method # 'GET', 'POST' request.args.get('q') # query string ?q=value request.form['name'] # form POST data request.json # parsed JSON body ``` ### Request Properties | Command | Description | |---------|-------------| | `request.args` | Query string parameters | | `request.form` | Form POST data | | `request.json` | Parsed JSON body | | `request.files` | Uploaded files | | `request.headers` | HTTP headers | | `request.cookies` | Cookie values | ### Response Helpers ``` from flask import jsonify, redirect, make_response return jsonify({'status': 'ok'}) # JSON response return redirect(url_for('index')) # redirect resp = make_response('body', 200) resp.headers['X-Custom'] = 'value' ``` ### Session ``` from flask import session app.secret_key = 'your-secret-key' session['user_id'] = 42 uid = session.get('user_id') ``` ## Forms ### WTForms Integration ``` pip install flask-wtf from flask_wtf import FlaskForm from wtforms import StringField, PasswordField from wtforms.validators import DataRequired ``` ### Define Form ``` class LoginForm(FlaskForm): username = StringField('User', validators=[DataRequired()]) password = PasswordField('Pass', validators=[DataRequired()]) ``` ### Use in View ``` @app.route('/login', methods=['GET', 'POST']) def login(): form = LoginForm() if form.validate_on_submit(): user = form.username.data return redirect(url_for('dashboard')) return render_template('login.html', form=form) ``` ### Form in Template ``` ``` ## Database ### SQLAlchemy Setup ``` pip install flask-sqlalchemy from flask_sqlalchemy import SQLAlchemy app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///app.db' db = SQLAlchemy(app) ``` ### Define Model ``` class User(db.Model): id = db.Column(db.Integer, primary_key=True) name = db.Column(db.String(80), nullable=False) email = db.Column(db.String(120), unique=True) posts = db.relationship('Post', backref='author') ``` ### CRUD Operations ``` user = User(name='Alice', email='alice@example.com') db.session.add(user) db.session.commit() User.query.filter_by(name='Alice').first() db.session.delete(user) db.session.commit() ``` ### Common Queries | Command | Description | |---------|-------------| | `Model.query.all()` | All records | | `Model.query.get(id)` | By primary key | | `.filter_by(name='X')` | Simple equality filter | | `.filter(Model.age > 18)` | Expression filter | | `.order_by(Model.name)` | Sort results | | `.limit(10).offset(20)` | Paginate results | ## Blueprints ### Create Blueprint ``` from flask import Blueprint blog = Blueprint('blog', __name__, url_prefix='/blog') @blog.route('/') def index(): return render_template('blog/index.html') ``` ### Register Blueprint ``` # app.py from blog import blog app.register_blueprint(blog) ``` ### Blueprint URL Building ``` url_for('blog.index') # => '/blog/' url_for('blog.post', id=5) # => '/blog/post/5' ``` ### Blueprint Structure | Command | Description | |---------|-------------| | `url_prefix` | Prefix all routes in blueprint | | `template_folder` | Custom template directory | | `static_folder` | Blueprint-specific static files | | `@bp.before_request` | Run before each blueprint request | ## Error Handling ### Custom Error Pages ``` @app.errorhandler(404) def not_found(e): return render_template('404.html'), 404 @app.errorhandler(500) def server_error(e): return render_template('500.html'), 500 ``` ### Abort Requests ``` from flask import abort @app.route('/admin') def admin(): if not current_user.is_admin: abort(403) return render_template('admin.html') ``` ### Custom Exceptions ``` from werkzeug.exceptions import HTTPException class InsufficientFunds(HTTPException): code = 402 description = 'Insufficient funds' ``` ### Logging ``` app.logger.info('User %s logged in', username) app.logger.warning('Disk space low') app.logger.error('Payment failed: %s', err) ``` ## Configuration ### Config Methods ``` app.config['DEBUG'] = True app.config.from_object('config.ProductionConfig') app.config.from_envvar('APP_SETTINGS') ``` ### Config Class Pattern ``` class Config: SECRET_KEY = os.environ.get('SECRET_KEY') SQLALCHEMY_TRACK_MODIFICATIONS = False class DevConfig(Config): DEBUG = True SQLALCHEMY_DATABASE_URI = 'sqlite:///dev.db' ``` ### Common Settings | Command | Description | |---------|-------------| | `SECRET_KEY` | Session signing key (required) | | `DEBUG` | Enable debug mode | | `TESTING` | Enable test mode | | `SQLALCHEMY_DATABASE_URI` | Database connection string | | `MAX_CONTENT_LENGTH` | Max upload size in bytes | | `JSON_SORT_KEYS` | Sort JSON output keys | ## Extensions ### Popular Extensions | Command | Description | |---------|-------------| | `Flask-SQLAlchemy` | ORM integration | | `Flask-Migrate` | Alembic database migrations | | `Flask-WTF` | Form handling with CSRF | | `Flask-Login` | User session management | | `Flask-Mail` | Email sending | | `Flask-CORS` | Cross-origin resource sharing | | `Flask-RESTful` | REST API building | | `Flask-Caching` | Response and function caching | ### Flask-Login ``` from flask_login import LoginManager, login_required login_manager = LoginManager(app) login_manager.login_view = 'login' @login_manager.user_loader def load_user(user_id): return User.query.get(int(user_id)) ``` ### Flask-Migrate ``` from flask_migrate import Migrate migrate = Migrate(app, db) # flask db init (once) # flask db migrate -m "add users" # flask db upgrade ```