Flask-admin无法在uWSGI和Nginx下导入名称_class_resolver

Question

I've hosted a python Flask project in AWS using uWSGI and Nginx , initially everything worked fine, but then I decided to implement Flask-admin module. Now page is showing me an Internal Server Error message.

I've checked UWSGI log and says:

  Traceback (most recent call last):
  File "/var/www/project/run.py", line 2, in <module>
    from app import app
  File "/var/www/project/app/__init__.py", line 22, in <module>
    from app import views
  File "/var/www/project/app/views.py", line 15, in <module>
    from flask_admin.contrib.sqla import ModelView
  File "/var/www/project/venv/local/lib/python2.7/site-packages/flask_admin/contrib/sqla/__init__.py", line 2, in <module>
    from .view import ModelView
  File "/var/www/project/venv/local/lib/python2.7/site-packages/flask_admin/contrib/sqla/view.py", line 17, in <module>
    from flask_admin.contrib.sqla.tools import is_relationship
  File "/var/www/project/venv/local/lib/python2.7/site-packages/flask_admin/contrib/sqla/tools.py", line 4, in <module>
    from sqlalchemy.ext.declarative.clsregistry import _class_resolver
ImportError: cannot import name _class_resolver
Fri Nov 10 07:41:08 2017 - unable to load app 0 (mountpoint='project.domain.com|') (callable not found or import error)
Fri Nov 10 07:41:08 2017 - --- no python application found, check your startup logs for errors ---

My app/views.py file:

# -*- coding: utf-8 -*-
import os, time
import flask
from flask import render_template, request, jsonify, send_from_directory, send_file, session, redirect, g, url_for, make_response, session, request
from flask_sqlalchemy import SQLAlchemy
from models import RequestsLog
import requests
import json
from app import app, db, appname #Line edited
import flask_admin as admin
#from flask_admin import Admin, helpers, expose  ------Line Erased
from flask_admin.contrib.sqla import ModelView
#from flask_admin.contrib import sqla   ----------- Line Erased

class MyAdminIndexView(admin.AdminIndexView):
        @admin.expose('/')
        def index(self):
                count = 56
                return self.render('admin/index.html', count=count)

class CustomView(ModelView):
       can_create = False
       can_edit = False
       can_delete = False  # disable model deletion
       can_export = True
       export_columns = ['phone_number', 'doc_number', 'date', 'doc_type']
       can_view_details = True       
       column_export_exclude_list = ['job_id', 'status']
       list_template = 'list.html'
       details_modal = True
       page_size = 50
       can_set_page_size = True

admin = admin.Admin(app, appname, template_mode='bootstrap3', index_view=MyAdminIndexView())
admin.add_view(CustomView(RequestsLog, db.session, menu_icon_type ='fa', menu_icon_value='fa-calendar'))

EDIT: If I comment this line everything works as it should:

from flask_admin.contrib.sqla import ModelView

Googling I've noticed that ImportError could be caused due a redundant import, but I think that all flask-admin imports are necessary.

I think the problem is not caused by uWSGI, but here is config file:

[uwsgi]
uid = www-data
gid = www-data

plugins=python
vhost=true
socket=/tmp/project.sock
chmod-socket = 666
chown-socket = www-data:www-data

enable-threads = true
procname-prefix = project_
chdir = /var/www/project

I've created again the virtual environment but everything installed prefectly.

EDIT: It should be noted that when I execute the run.py file directly everything work as expected, even with flask-admin imports, the problem is just under uWSGI.

Finally my project is hosted in an Ubuntu 14.04 Trusty AWS dependency.

EDIT:

Adding run.py file:

#!venv/bin/python
from app import app

if __name__ == '__main__':

    app.run(host='0.0.0.0', threaded=True, debug=True, port=5010)

EDIT 2:

Adding __init__.py file:

# -*- coding: utf-8 -*-
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_socketio import SocketIO
#import flask_admin as admin
from flask_babel import Babel

#I've defined the next three lines to avoid encoding conflicts
import sys
reload(sys)
sys.setdefaultencoding("utf-8")

app = Flask(__name__)
app.config.from_pyfile('../config.py')
appname = 'Project'

db = SQLAlchemy(app, session_options={'autoflush': False})
babel = Babel(app)

async_mode = None
socketio = SocketIO(app, async_mode=async_mode, ping_timeout=1800)

from app import views

venv/bin/pip freeze output:

Babel==2.5.1
Flask==0.12.2
Flask-Admin==1.5.0
Flask-Babel==0.11.2
Flask-SQLAlchemy==2.3.2
Flask-SocketIO==2.9.2
Jinja2==2.9.6
MarkupSafe==1.0
SQLAlchemy==1.1.14
WTForms==2.1
Werkzeug==0.12.2
argparse==1.2.1
asn1crypto==0.22.0
certifi==2017.7.27.1
chardet==3.0.4
click==6.7
click-completion==0.2.1
colorama==0.3.9
crayons==0.1.2
elibom==1.2
gunicorn==19.7.1
idna==2.6
itsdangerous==0.24
psycopg2==2.7.3.1
py==1.4.34
pycycle==0.0.8
pytest==3.2.3
python-engineio==1.7.0
python-socketio==1.8.1
pytz==2017.3
requests==2.18.4
six==1.11.0
uWSGI==2.0.15
urllib3==1.22
wsgiref==0.1.2

Nginx config file:

server {
        listen  80 ssl;
    server_name project.domain.com; 
    listen 443 ssl;

        ssl_certificate /etc/nginx/project.domain.com/fullchain.pem;
        ssl_certificate_key /etc/nginx/project.domain.com/privkey.pem;
        ssl_session_timeout 1d;
        ssl_session_cache shared:SSL:50m;

        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS !MEDIUM !RC4";
        ssl_prefer_server_ciphers on;

        ssl_stapling on;
        ssl_stapling_verify on;

    location /static/ {
                autoindex               on;
                alias                   /var/www/project/app/static/;
        }

        location / {
                include                 uwsgi_params;
                uwsgi_pass              unix:/tmp/project.sock;
                uwsgi_read_timeout      300;
                uwsgi_param             UWSGI_PYHOME    /var/www/project/venv;
                uwsgi_param             UWSGI_CHDIR     /var/www/project;
                uwsgi_param             UWSGI_MODULE    run;
                uwsgi_param             UWSGI_CALLABLE  app;
        }

        error_page 404 /404.html;

        location /socket.io {
            include proxy_params;
            proxy_http_version 1.1;
            proxy_buffering off;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "Upgrade";
            proxy_pass http://127.0.0.1:5010/socket.io;
    }
}

Thanks a lot for your help.

Answer 1

I'll start with the immediate error, and then address reasons it might run differently from uWSGI.

Immediate error:

There's a little bit of fast-and-loose importing going on here, which is common as you're building something out or tacking on features. A good place to start is by paring it back to reduce redundancy and cleaning it up. Even if you don't fix the issue on the way, every deletion will make the remaining imports easier to think through.

In addition to the change Luis suggested in the comments, you can combine the following two lines which use different conventions to import from the same place:

import flask_admin as admin from flask_admin import Admin, helpers, expose

You could either just import flask_admin itself:

import flask_admin
...
class MyAdminIndexView(flask_admin.AdminIndexView):
    @flask_admin.expose('/')
...
admin = flask_admin.Admin...

Or you could add AdminIndexView to the list of explicit imports:

from flask_admin import Admin, AdminIndexView, helpers, expose

Most Flask import errors I've run into come from trying to import another object from within your app that hasn't been properly initialized yet (often because of a circular import), so I'd guess it's lurking behind one of these 3 lines:

from app import app
...
from models import RequestsLog
...
from app import db, appname

The cause may leap out at you, but sometimes the cycle is several modules deep. If it doesn't, a good next step is running your project through a linter such as Pylint . If you're already running Pylint, you may want to double-check your settings and make sure import-related rules are enabled! If you aren't already using it, you can run it manually from the command line, or use it via an editor plugin that may run your code through it as you work (it's likely to gripe about many issues if you weren't already linting--but I'd just focus on import/module-related codes to start).

You might also be able to ferret it out quicker with a more focused tool like pycycle , though I've yet to try it out myself.

Sometimes you'll be able to avoid circular import issues in Flask with careful design or a little refactoring, but as the complexity of your app grows, new features are more likely to lead you into a circular import. If you identify the problem but can't find an obvious way around, the next step is typically to adopt the Flask application factory pattern . This pattern isn't magic--you can still paint yourself into import-issue corners--but it does tend to make them easier to refactor yourself out of.

Running from uWSGI:

For now, I think these are the two most-likely reasons you'd see different behavior when running via python from command-line, and from behind uWSGI. This is just a generic list, since a more specific answer hinges on a few things outside of the files you've included. uWSGI and nginx both have a fairly complex array of configuration options and I'm flying from memory here, so I might not be completely accurate... :)

1. The app is set up conditionally, perhaps using environment variables or something like if __name__ == '__main__': , so you aren't running exactly the same code in both cases.

2. uWSGI isn't using the same callable/module. Between the config file and command-line, I recall there being a handful of ways to tell uWSGI which python file to load, and what the name of the application's python object is, but I don't recall being able to do so from nginx. I don't see any of these specified in your config file, but the log snippet you include suggests the import exception occurs while uWSGI is trying to find a callable (typically app or application ) in run.py . Are you running the same code from the CLI?