Flask

Databases

Flask-SQLAlchemy, an extension that provides a Flask-friendly wrapper to the popular SQLAlchemy package, which is an Object Relational Mapper or ORM.

pip install flask-sqlalchemy

Flask-Migrate a database migration framework for SQLAlchemy.

pip install flask-migrate

Configurations

Most important configuration is SQLALCHEMY_DATABASE_URI

Models

app/models.py

from datetime import datetime
from app import db

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    password_hash = db.Column(db.String(128))
    posts = db.relationship('Post', backref='author', lazy='dynamic')

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

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.String(140))
    timestamp = db.Column(db.DateTime, index=True, default=datetime.utcnow)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

    def __repr__(self):
        return '<Post {}>'.format(self.body)

Database Migrations

Note

FLASK_APP should be set for these commands to execute properly

Create the migrations repository

flask db init

Generate automatic migrations

flask db migrate

We can provide a -m optional argument that adds a short descriptive text to the migration.

flask db migrate -m "added users table"

Upgrade

The flask db migrate command does not make any changes to the database, it just generates the migration script. To apply the changes to the database, the flask db upgrade command must be used.

Downgrade

flask db downgrade

Blueprints

Blueprints is the way flask achieve modularity, reusablity and extensibility.

Here's a simple hello world blueprint.

.
├── app
│   ├── hello
│   │   ├── __init__.py
│   │   ├── routes.py
│   │   └── templates
│   │       └── index.html
│   ├── __init__.py
├── flask_learning.py

app/__init__.py

from flask import Flask
from app.hello import blueprint

def create_app():
    app = Flask(__name__)
    app.register_blueprint(blueprint)
    return app

app/hello/__init__.py

from flask import Blueprint

blueprint = Blueprint('hello_blueprint', __name__, template_folder='templates')

from app.hello import routes

app/hello/routes.py

from flask import render_template

from app.hello import blueprint


@blueprint.route("/<string:name>")
def index(name):
    return render_template("index.html", name=name)

flask_learning.py

from app import create_app

app = create_app()

Configuring the app

Flask provides many ways of loading configurations to the application.

One way is to define a configuration object and load configuration from there using app.config.from_object method.

Note

from_object loads only the uppercase attributes of the module/class

Using objects for configuration we can separate different configuration for developpement, testing and production

class Config(object):
    SOME_CONFIG = 'default-value'

class Devlopement(Config):
    pass

class Production(Config):
    SOME_CONFIG = 'production-value'

class Testing(Config):
    SOME_CONFIG = 'testing-config'

Jinja

Basic

template_dir = os.path.dirname(template_path)
template_name = os.path.basename(template_path)
env = Environment(loader=FileSystemLoader(template_dir), **env_kwargs)
template = env.get_template(template_name)

Raise exception

def raise_from_jinja(*args, **kwargs):
    raise Exception(*args, **kwargs)

# And add it to the globals of the template
template.globals.update({'raise': raise_from_jinja})