Flask AttributeError: module object has no attribute 'app' - python-3.x

Question

I am a newbie to flask, and I had below import related AttributeError when flask run in flask_lab folder.
Any help would be appreciated.
Working Directory:
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!')
Error:
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'
Got complaints from stackoverflow about too many code and not enough details.... more words, more words....

Answer 1

While interpreting app.py if the python interpreter encounters a line import flask_lab.tmp.test.demo then it will start interpreting tmp/test.py immediately. But tmp/test.py again imports flask_lab.
At this point, since the interpreter had already encountered flask_lab, it will start searching for app in that namespace. But it had never reached till that line. Since you had already imported tmp.test before app was defined in the module, there is no flask_lab.app yet and hence the error.
And if you had called tmp.test directly, you would have encountered a circular import error as well.
So the way out is to avoid the circular import scenario. Move the db object to a separate module and call it in both these modules. Flask-SQLAlchemy provides a method called init_app which is meant for such use case.
Lets create a module called common which will contain the common variables.
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!')
Note that I have also replaced the flask_lab imports with relative imports. They are cleaner. The code inside the package should avoid using the package name in the imports. That way if you change the package name later, you can do so without having to change all the code inside.