ImportError：无法从“模块”导入名称“变量”

Question

I am using Flask for my web framework. I am having an issue with imports. I am not understanding why can't I import my variable when I declare it within my my_app/__init__.py :

from flask import Flask
from flask_login import LoginManager
from my_app.some_module.my_class.py import auth


app = Flask(__name__)
login_manager = LoginManager()

class Config:
    def __init__(self):
        pass

config = Config()

My conflictuous imports are present in my_app/some_module/my_class.py :

from flask import Blueprint
from my_app import login_manager  # this one works fine
from my_app import config

auth = Blueprint('auth', __name__)

I run the app with run.py :

from my_app import app
app.run(debug=True)

I then get the error:

Traceback (most recent call last):
  ...
  File ".../my_app/some_module/my_class.py", line 1, in <module>
    from my_app import login_manager, config
ImportError: cannot import name 'config' from 'my_app' (.../my_app/__init__.py)

Project structure is:

my_app
  + __init__.py
  some_module
    + __init__.py
    + my_class.py
+ run.py

Answer 1

The problem is that you have a cyclic dependency. By the time you import auth from my_app.some_module.my_class.py your config is not set yet. Try moving that import to the end of the my_app/__init__.py file like:

from flask import Flask
from flask_login import LoginManager


app = Flask(__name__)
login_manager = LoginManager()

class Config:
    pass

config = Config()

from my_app.some_module.my_class.py import auth

Answer 2

You have a cyclic import: my_app.some_module -> my_app.some_module.my_class -> my_app.some_module .

You can fix this by moving both Config and config to a separate module my_app.some_module.config .

# my_app.some_module.my_config
class Config:
    pass

config = Config()

# my_app.some_module.my_class
from .my_config import config

# my_app.some_module.__init__
from .my_config import config
from .my_class import MyClass

This means that every import does not depend on previous imports:

my_app.some_module
|-> my_app.some_module.my_class -> my_app.some_module.config
\-> my_app.some_module.my_config

Doing imports this way instead of moving the import for .my_class to the end of __init__.py is more robust. You can freely reorder the imports of .my_class and .my_config at the top of files.