Using sqlalchemy with flaskmysql : error Sqlalchemy engine=None

Question

from flask import Flask
from flask_migrate import Migrate
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)

db = SQLAlchemy()
migrate = Migrate()

i have set app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql+mysqldb://~'

db.init_app(app) migrate.init_app(app,db)

when i print db i get

anyone could please tell me how to give uri to sqlalchemy engine? i tried create_engine with that uri and checked it was working..