python-3.x - Flask属性错误: module object has no attribute 'app'

Question

我是flask的新手，当flask_lab文件夹中的flask run时，我遇到了以下导入相关的AttributeError。

如有任何帮助，我们将不胜感激。

工作目录:

flask_lab
├── __init__.py
├── Pipfile
├── Pipfile.lock
├── README.md
├── tmp
│   ├── __init__.py
│   └── test.py
└── app.py

flask_lab/app.py:

import os
import click

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

import flask_lab.tmp.test.demo

app = Flask(__name__)
prefix = 'sqlite:////'
app.config['SQLALCHEMY_DATABASE_URI'] = prefix + os.path.join(app.root_path, 'data.db')
db = SQLAlchemy(app)

class User(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    name = db.Column(db.String(20))

flask_lab.tmp.test.demo()

flask_lab/tmp/test.py:

import flask_lab.app

print(flask_lab.app.db)


def demo():
    print('yeah!')

错误:

Traceback (most recent call last):
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/bin/flask", line 10, in <module>
    sys.exit(main())
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 894, in main
    cli.main(args=args, prog_name=name)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 557, in main
    return super(FlaskGroup, self).main(*args, **kwargs)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 767, in run_command
    app = DispatchingApp(info.load_app, use_eager_loading=eager_loading)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 293, in __init__
    self._load_unlocked()
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 317, in _load_unlocked
    self._app = rv = self.loader()
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 377, in load_app
    raise_if_not_found=False)
  File "/home/huafeng/.local/share/virtualenvs/flask_lab-x_qc9OjH/lib/python3.6/site-packages/flask/cli.py", line 235, in locate_app
    __import__(module_name)
  File "/home/huafeng/Desktop/flask_lab/app.py", line 7, in <module>
    import flask_lab.tmp.test.demo
  File "/home/huafeng/Desktop/flask_lab/tmp/test.py", line 3, in <module>
    print(flask_lab.app.db)
AttributeError: module 'flask_lab' has no attribute 'app'

从 stackoverflow 收到关于代码过多且细节不足的投诉...更多字，更多字...

Answer 1

在解释 app.py 时，如果 python 解释器遇到一行 import Flask_lab.tmp.test.demo 那么它将立即开始解释 tmp/test.py。但 tmp/test.py 再次导入flask_lab。

此时，由于解释器已经遇到了flask_lab，因此它将开始在该命名空间中搜索app。但它从未达到那条线。由于您在模块中定义 app 之前已经导入了 tmp.test，因此尚不存在 flask_lab.app，因此会出现错误。

如果您直接调用 tmp.test，您也会遇到循环导入错误。

所以出路是避免循环导入场景。将 db 对象移至单独的模块并在这两个模块中调用它。 Flask-SQLAlchemy 提供了一个名为 init_app 的方法，该方法适用于此类用例。

让我们创建一个名为 common 的模块，它将包含公共(public)变量。

flask_lab/common.py

from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()

flask_lab/app.py:

import os
import click

from flask import Flask
from .common import db
from .tmp.test import demo

class User(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    name = db.Column(db.String(20))

app = Flask(__name__)
prefix = 'sqlite:////'
app.config['SQLALCHEMY_DATABASE_URI'] = prefix + os.path.join(app.root_path, 'data.db')
db.init_app(app)

demo()

flask_lab/tmp/test.py:

from .common import db

print(db)


def demo():
    print('yeah!')

请注意，我还用相对导入替换了 flask_lab 导入。他们更干净。包内的代码应避免在导入中使用包名称。这样，如果您稍后更改包名称，则无需更改其中的所有代码即可执行此操作。