Attribute导入烧瓶SQLAlchemy模型时出错

Question

我将按照本教程构建一个基于JWT的身份验证系统。

app.py:

from flask import Flask
from flask_restful import Api
from flask_sqlalchemy import SQLAlchemy
app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///app.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.config['SECRET_KEY'] = 'd0cad49580952003e6ae01499c7bb190a4b4f9a5babd866f47064707f7b78506'
api = Api(app)
db = SQLAlchemy(app)

@app.before_first_request
def create_tables():
db.create_all()

import resources, models
api.add_resource(resources.UserRegistration, '/registration')

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

models.py:

from app import db

class UserModel(db.Model):
__tablename__ = 'users'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(100), nullable=False)
password = db.Column(db.String(150), nullable=False)
email = db.Column(db.String(100), unique=True, nullable=False)
def __init__(self, name, password, email):
self.name = name
self.password = password
self.email = email
@classmethod
def find_by_username(cls, username):
return cls.query.filter_by(username=username).first()
def save_to_db(self):
db.session.add(self)
db.session.commit()

resources.py:

from flask_restful import Resource, reqparse
from models import UserModel

class UserRegistration(Resource):
def post(self):
parser = reqparse.RequestParser()
parser.add_argument('name', help='This field cannot be blank', required=True)
parser.add_argument('email', help='This field cannot be blank', required=True)
parser.add_argument('password', help='This field cannot be blank', required=True)
data = parser.parse_args()
if UserModel.find_by_username(data['name']):
return {'message': 'User {} already exists'.format(data['name'])}
new_user = UserModel(
name=data['name'],
password=data['password'],
email=data['email']
)
try:
new_user.save_to_db()
return {
'status': 'User {} was created'.format(data['username'])}
except:
return {'message': 'Something went wrong'}, 500

当我运行app.py时，我得到以下错误：

Traceback (most recent call last):
File "G:pythonPycharmProjectsvumonicapp.py", line 19, in <module>
import resources, models
File "G:pythonPycharmProjectsvumonicresources.py", line 2, in <module>
from models import UserModel
File "G:pythonPycharmProjectsvumonicmodels.py", line 1, in <module>
from app import db
File "G:pythonPycharmProjectsvumonicapp.py", line 21, in <module>
api.add_resource(resources.UserRegistration, '/registration')
AttributeError: module 'resources' has no attribute 'UserRegistration'

当我从resources.py.中删除from models import UserModel时，此错误消失

我想不出出错的原因。

我使用的是Flask==1.1.2，Flask SQLAlchemy==2.4.4和Flask RESTful==0.3.8

这是我第一次开发API，所以任何帮助都将不胜感激。

Answer 1

您正面临循环导入问题。

当Python导入模块时，它会检查模块注册表以查看该模块是否已经导入。如果模块已经注册，Python将使用缓存中的现有对象。模块注册表是已按模块名称初始化和索引的模块表。此表可通过sys.modules访问。

如果它没有注册，Python会找到模块，在必要时对其进行初始化，并在新模块的命名空间中执行它。

要了解更多关于循环导入的信息，您可以阅读文章：

https://stackabuse.com/python-circular-imports/

https://www.stefaanlippens.net/circular-imports-type-hints-python.html

Miguel Grinberg的这个教程是一个生命救世主https://www.youtube.com/watch?v=NH-8oLHUyDc&t=3205s