Why I can't move import at top?

Question

I am learning Flask. I found example with follow code

__init__.py:

from flask import Flask

app = Flask(__name__)
from app import views

view.py:

from app import app

@app.route('/')
def index():
    return "hello world"

and run.py (on top level):

from app import app
app.run()

I can't understand why I can't move from app import views to the top. If I do so I am getting error:

> run.py
Traceback (most recent call last):
  File "F:\app1\run.py", line 1, in <module>
    from app import app
  File "F:\app1\app\__init__.py", line 2, in <module>
    from app import views
  File "F:\app1\app\views.py", line 1, in <module>
    from app import app
ImportError: cannot import name 'app'

I think this is a relative import vs absolute import problem in python 3. — Jean-François Fabre, Oct 19 '17 at 19:36
check this Q&A and tell me if it solves it: https://stackoverflow.com/questions/38564382/importerror-on-python-3-worked-fine-on-python-2-7 — Jean-François Fabre, Oct 19 '17 at 19:37
I believe this is a circular-import problem: `app` object has to be defined in `__init__.py` before it can be imported from `view.py`. — randomir, Oct 19 '17 at 19:47

kvorobiev · Answer 1 · 2017-10-19T20:01:35.060

In run.py file line

from app import app

means "from package app (folder with __init__.py file, F:\app1\app in your case, import object app", and in __init__.py file line

app = Flask(__name__)

creates an application object. Main confusion caused by your application name - app - which coincides with application object name.
If you move line

from app import views

above

app = Flask(__name__)

from app import app in view.py will raise ImportError, because object app from app package not defined yet.

score 0 · Answer 2 · answered Oct 19 '17 at 19:51

Python is interpreted language. When it comes to one line it try to execute it. And that is why you cannot import views before defining app.

Working version of __init__.py

app = Flask(__name__)
from app import views # it will "execute: views.py"
# views.py - indenting for better understanding
    from app import app # App is defined here and it can be imported

Not working version of __init__.py

from app import vies # it will execute vies.py"
# vies.py - indenting for better understanding
    from app import app # Exception - app doesn't exist here
app = Flask(__name__)

You need to be really careful with python imports.

Why I can't move import at top?

2 Answers2