Python 未设置sqlalchemy数据库uri

即使设置了数据库url，也会出现错误。错误消息如下所示

Neither SQLALCHEMY_DATABASE_URI nor SQLALCHEMY_BINDS is set. Defaulting SQLALCHEMY_DATABASE_URI to "sqlite:///:memory:"

这是我的app.py

from flask import Flask, Blueprint
from flask_alembic import Alembic
from flask_restplus import Resource
from werkzeug.contrib.fixers import ProxyFix
from flask_sqlalchemy import SQLAlchemy
from config import config_by_name
from .apis import api

db = SQLAlchemy()


def create_app(config_name):
    app = Flask(__name__)
    app.config.from_object(config_by_name[config_name])
    app.wsgi_app = ProxyFix(app.wsgi_app)
    app.config['BUNDLE_ERRORS'] = True
    db.init_app(app)
    api.init_app(app)

    alembic = Alembic()
    alembic.init_app(app)

    blueprint = Blueprint('api', __name__, url_prefix='/api')
    app.register_blueprint(blueprint)

    return app

当我运行create_app（）时，它会获取配置名称并设置配置。这是我的config.py

class DevelopmentConfig(Config):
    """
    config for develop
    """
    DEBUG = True
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'mysql://root:dev@localhost:3306/develop'
    PRESERVE_CONTEXT_ON_EXCEPTION = False
    SQLALCHEMY_TRACK_MODIFICATIONS = False


config_by_name = dict(
    dev=DevelopmentConfig    )

还有其他环境配置，但我已将其删除。 这是run.py

app = create_app('dev')

if __name__ == '__main__':
    from config import SERVER_HOST, SERVER_PORT
    app.run(host=SERVER_HOST, port=SERVER_PORT, debug=True)