python packaging causes importerror

Question

I am getting an annoying import error when I try to import a variable in an __init__.py file. I have attached the files involved and my directory structure:

#/home/me/app/app/__init__.py
from flaskext.sqlalchemy import SQLAlchemy

db = SQLAlchemy(app)

#/home/me/app/app/models/__init__.py
from datetime import datetime
from app import db

#shell
[me@archlinux app]$ pwd
/home/me/app
[me@archlinux app]$ ./manage.py 
/home/me/app/app/__init__.pyc
Traceback (most recent call last):
  File "./manage.py", line 7, in <module>
    from app import app
  File "/home/me/app/app/__init__.py", line 3, in <module>
    from app.views.post import post
  File "/home/me/app/app/views/post.py", line 4, in <module>
    from app.models import Post
  File "/home/me/app/app/models/__init__.py", line 5, in <module>
    from app import db
ImportError: cannot import name db

[me@archlinux app]$ tree 
.
├── apikey.txt
├── manage.py
├── app
│   ├── forms
│   │   ├── __init__.py
│   │   └── __init__.py~
│   ├── __init__.py
│   ├── __init__.py~
│   ├── __init__.pyc
│   ├── models
│   │   ├── __init__.py
│   │   ├── __init__.py~
│   │   └── __init__.pyc
│   ├── static
│   │   ├── css
│   │   │   └── style.css
│   │   ├── images
│   │   │   ├── favicon.png
│   │   │   ├── logo.png
│   │   │   ├── text_logo.png
│   │   │   ├── thumb_down_active.png
│   │   │   ├── thumb_down_inactive.png
│   │   │   ├── thumb_up_active.png
│   │   │   └── thumb_up_inactive.png
│   │   ├── js
│   │   │   └── index.js
│   │   └── sitemap.xml
│   ├── templates
│   │   ├── 404.html
│   │   ├── 500.html
│   │   ├── about.html
│   │   ├── base.html
│   │   ├── feedback
│   │   │   └── feedback_form.html
│   │   ├── form.html
│   │   ├── posts
│   │   │   ├── comment.html
│   │   │   ├── post.html
│   │   │   └── posts.html
│   │   ├── spam.html
│   │   ├── terms.html
│   │   └── users
│   │       ├── login_form.html
│   │       └── sign_up_form.html
│   ├── util
│   │   ├── forms.py
│   │   ├── honeypot.py
│   │   ├── __init__.py
│   │   ├── __init__.py~
│   │   ├── json_http.py
│   │   ├── models.py
│   │   └── spam.py
│   └── views
│       ├── feedback.py
│       ├── __init__.py
│       ├── __init__.pyc
│       ├── post.py
│       ├── post.pyc
│       └── user.py
├── settings.py
├── settings.pyc
└── TiddlyWiki.html

13 directories, 49 files

What might be the problem?

Some playing around with pdb_trace() placed right before the import:

(Pdb) import app
(Pdb) app
<module 'app' from '/home/ramin/app/app/__init__.pyc'>
(Pdb) dir(app)
['Flask', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', 'views']

no db in app :)

Answer 1

There is likely a problem in the app/__init__.py, that raises an error there, somehow, perhaps a syntax error. These errors have a tendency to be hidden as import errors later.

Put a

import pdb;pdb.set_trace()

At the beginning of the module that it tries to import. You can then step through that module to see what the real error is.

Answer 2

This is most often caused by Python finding a package or module with the same name that is different than the one you think it is. I can also happen when you try to run it from withing the package space. First try to change default directory to home, and try again. If it still fails, try adding to the top of your modules:

from __future__ import absolute_import

That prevents the old default behavior of using relative imports from the package space.

Also, in the debugger session, print out the __file__ attribute and check that it is what you expect.

Answer 3

"app/app" is asking for trouble, it both directories are in the search path.

Answer 4

From the stacktrace it seems the following is going on:

• inside app you import views.post
• this imports Post from models
• the models import db from app
• which will import app, which will import views...

so, looks like a cyclic import?

Answer 5

I was seeing "ImportError: No module named" because my modules didn't have execute permissions.