from flask import Flask
from flaskext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/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)
    email = db.Column(db.String(120), unique=True)

    def __init__(self, username, email):
        self.username = username
        self.email = email

    def __repr__(self):
        return '<User %r>' % self.username

pip install flask-sqlalchemy

>>> from flaskext.sqlalcemy import SQLAlchemy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named flaskext.sqlalcem