使用 flask 框架开发 web 应用，初始化数据库的代码放在哪里最合适？

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from flask import Flask, render_template
from www.api import api_urls
from mvc import db

app = Flask(__name__)
app.config.from_object('config.config')
app.register_blueprint(api_urls.api_bp)
#init db
db.create_engine('dbuser', 'dbpasswd', 'dbname')

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

@app.route('/test')
def test():
    print hash(db)  #与api_urls.py的hash结果不同
    print hash(db.engine) #db.engine不为None
    return render_template('test.html')

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

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from flask import Blueprint, render_template, request, jsonify
from www.mvc import db

api_bp = Blueprint('api_bp', __name__)
#不再初始化一次数据库则不能访问
# db.create_engine('dbuser', 'dbpasswd', 'dbname')

@api_bp.route('/blogs')
def blogs_list():
    print hash(db) #与wsgiapp.py的hash结果不同
    print hash(db.engine) #不在文件中初始化的话，db.engine为None
    return 'blogs'

└─www
    ├─api --->api_urls.py
    ├─config
    ├─mvc --->db.py
    └─wsgiapp.py