Flask, blueprints uses celery task and got cycle import - import

Question

I have an application with Blueprints and Celery
the code is here:
config.py
import os
from celery.schedules import crontab
basedir = os.path.abspath(os.path.dirname(__file__))
class Config:
SECRET_KEY = os.environ.get('SECRET_KEY') or ''
SQLALCHEMY_COMMIT_ON_TEARDOWN = True
RECORDS_PER_PAGE = 40
SQLALCHEMY_DATABASE_URI = ''
CELERY_BROKER_URL = ''
CELERY_RESULT_BACKEND = ''
CELERY_RESULT_DBURI = ''
CELERY_TIMEZONE = 'Europe/Kiev'
CELERY_ENABLE_UTC = False
CELERYBEAT_SCHEDULE = {}
#staticmethod
def init_app(app):
pass
class DevelopmentConfig(Config):
DEBUG = True
WTF_CSRF_ENABLED = True
APP_HOME = ''
SQLALCHEMY_DATABASE_URI = 'mysql+mysqldb://...'
CELERY_BROKER_URL = 'sqla+mysql://...'
CELERY_RESULT_BACKEND = "database"
CELERY_RESULT_DBURI = 'mysql://...'
CELERY_TIMEZONE = 'Europe/Kiev'
CELERY_ENABLE_UTC = False
CELERYBEAT_SCHEDULE = {
'send-email-every-morning': {
'task': 'app.workers.tasks.send_email_task',
'schedule': crontab(hour=6, minute=15),
},
}
class TestConfig(Config):
DEBUG = True
WTF_CSRF_ENABLED = False
TESTING = True
SQLALCHEMY_DATABASE_URI = 'mysql+mysqldb://...'
class ProdConfig(Config):
DEBUG = False
WTF_CSRF_ENABLED = True
SQLALCHEMY_DATABASE_URI = 'mysql+mysqldb://...'
CELERY_BROKER_URL = 'sqla+mysql://...celery'
CELERY_RESULT_BACKEND = "database"
CELERY_RESULT_DBURI = 'mysql://.../celery'
CELERY_TIMEZONE = 'Europe/Kiev'
CELERY_ENABLE_UTC = False
CELERYBEAT_SCHEDULE = {
'send-email-every-morning': {
'task': 'app.workers.tasks.send_email_task',
'schedule': crontab(hour=6, minute=15),
},
}
config = {
'development': DevelopmentConfig,
'default': ProdConfig,
'production': ProdConfig,
'testing': TestConfig,
}
class AppConf:
"""
Class to store current config even out of context
"""
def __init__(self):
self.app = None
self.config = {}
def init_app(self, app):
if hasattr(app, 'config'):
self.app = app
self.config = app.config.copy()
else:
raise TypeError
init.py:
import os
from flask import Flask
from celery import Celery
from config import config, AppConf
def create_app(config_name):
app = Flask(__name__)
app.config.from_object(config[config_name])
config[config_name].init_app(app)
app_conf.init_app(app)
# Connect to Staging view
from staging.views import staging as staging_blueprint
app.register_blueprint(staging_blueprint)
return app
def make_celery(app=None):
app = app or create_app(os.getenv('FLASK_CONFIG') or 'default')
celery = Celery(__name__, broker=app.config.CELERY_BROKER_URL)
celery.conf.update(app.conf)
TaskBase = celery.Task
class ContextTask(TaskBase):
abstract = True
def __call__(self, *args, **kwargs):
with app.app_context():
return TaskBase.__call__(self, *args, **kwargs)
celery.Task = ContextTask
return celery
tasks.py:
from app import make_celery, app_conf
cel = make_celery(app_conf.app)
#cel.task
def send_realm_to_fabricdb(realm, form):
some actions...
and here is the problem:
The Blueprint "staging" uses task send_realm_to_fabricdb, so it makes: from tasks import send_realm_to_fabricdb
than, when I just run application, everything goes ok
BUT, when I'm trying to run celery: celery -A app.tasks worker -l info --beat, it goes to cel = make_celery(app_conf.app) in tasks.py, got app=None and trying to create application again: registering a blueprint... so I've got cycle import here.
Could you tell me how to break this cycle?
Thanks in advance.

Answer 1

I don't have the code to try this out, but I think things would work better if you move the creation of the Celery instance out of tasks.py and into the create_app function, so that it happens at the same time the app instance is created.
The argument you give to the Celery worker in the -A option does not need to have the tasks, Celery just needs the celery object, so for example, you could create a separate starter script, say celery_worker.py that calls create_app to create app and cel and then give it to the worker as -A celery_worker.cel, without involving the blueprint at all.
Hope this helps.

Answer 2

What I do to solve this error is that I create two Flask instance which one is for Web app, and another is for initial Celery instance.
Like #Miguel said, I have
celery_app.py for celery instance
manager.py for Flask instance
And in these two files, each module has it's own Flask instance.
So I can use celery.task in Views. And I can start celery worker separately.

Answer 3

Thanks Bob Jordan, you can find the answer from https://stackoverflow.com/a/50665633/2794539,
Key points:
1. make_celery do two things at the same time: create celery app and run celery with flask content, so you can create two functions to do make_celery job
2. celery app must init before blueprint register

Answer 4

Having the same problem, I ended up solving it very easily using shared_task (docs), keeping a single app.py file and not having to instantiate the flask app multiple times.
The original situation that led to the circular import:
from src.app import celery # src.app is ALSO importing the blueprints which are importing this file which causes the circular import.
#celery.task(bind=True)
def celery_test(self):
sleep(5)
logger.info("Task processed by Celery.")
The current code that works fine and avoids the circular import:
# from src.app import celery <- not needed anymore!
#shared_task(bind=True)
def celery_test(self):
sleep(5)
logger.info("Task processed by Celery.")
Please mind that I'm pretty new to Celery so I might be overseeing important stuff, it would be great if someone more experienced can give their opinion.