Celery

Common errors

Warnings

Install

Install compatibility verified last tested: 2026-05-12

Imports

• Celery
  wrong

  # Old Celery 4.x lowercase config — removed in Celery 5.0:
  app.config_from_object({
      'CELERY_TASK_SERIALIZER': 'json',     # removed — KeyError or ignored
      'CELERY_RESULT_BACKEND': 'redis://...',  # removed
      'CELERY_BROKER_URL': 'redis://...',      # removed
  })

  correct

  from celery import Celery
  
  app = Celery(
      'myapp',
      broker='redis://localhost:6379/0',
      backend='redis://localhost:6379/0'
  )
  
  # Correct: use @app.task decorator
  @app.task
  def add(x, y):
      return x + y
  
  # Call async
  result = add.delay(4, 6)
  print(result.get(timeout=10))

  Old uppercase CELERY_* config keys were deprecated in 4.0 and fully removed in 5.0. Use lowercase keys: task_serializer, result_backend, broker_url.

Quickstart verified last tested: 2026-04-23

Requires Redis running. Start worker in separate terminal: celery -A tasks worker --loglevel=info

# tasks.py
from celery import Celery

app = Celery(
    'tasks',
    broker='redis://localhost:6379/0',
    backend='redis://localhost:6379/0'
)

app.conf.update(
    task_serializer='json',
    accept_content=['json'],
    result_serializer='json',
    timezone='UTC',
)

@app.task
def add(x, y):
    return x + y

@app.task(bind=True, max_retries=3)
def fetch_data(self, url):
    try:
        import requests
        return requests.get(url).json()
    except Exception as exc:
        raise self.retry(exc=exc, countdown=5)

# --- Run worker ---
# celery -A tasks worker --loglevel=info

# --- Call from client ---
# result = add.delay(4, 6)
# print(result.get(timeout=10))  # 10