Checklist

• I have verified that the issue exists against the main branch of Celery.
• This has already been asked to the discussions forum first.
• I have read the relevant section in the contribution guide on reporting bugs.
• I have checked the issues list for similar or identical bug reports.
• I have checked the pull requests list for existing proposed fixes.
• I have checked the commit log to find out if the bug was already fixed in the main branch.
• I have included all related issues and possible duplicate issues in this issue.

Mandatory Debugging Information

• Celery version: 5.6.1
• Flower version: 2.0.1
• Python version: 3.12
• OS: Linux (Docker container)
• Broker: Redis
• Result backend: Redis

Description

When starting Flower with a Django/Celery application, an AttributeError is raised in celery/fixups/django.py at line 220.

The error occurs because self.worker.pool_cls is a string ('prefork') instead of a class object, so accessing .__module__ fails.

Error Traceback

File "/usr/local/lib/python3.12/site-packages/celery/fixups/django.py", line 220, in _close_database
    is_prefork = "prefork" in self.worker.pool_cls.__module__
                              │    │      └ 'prefork'
                              │    └ <Worker: celery@75efc8aa2596 (INIT)>
                              └ <celery.fixups.django.DjangoWorkerFixup object at 0x7f77b5998f20>

AttributeError: 'str' object has no attribute '__module__'. Did you mean: '__mod__'?

Full Stack Trace

File "/usr/local/lib/python3.12/site-packages/celery/fixups/django.py", line 116, in on_worker_init
    self.worker_fixup.install()
File "/usr/local/lib/python3.12/site-packages/celery/fixups/django.py", line 161, in install
    self.close_database()
File "/usr/local/lib/python3.12/site-packages/celery/fixups/django.py", line 207, in close_database
    return self._close_database()
File "/usr/local/lib/python3.12/site-packages/celery/fixups/django.py", line 220, in _close_database
    is_prefork = "prefork" in self.worker.pool_cls.__module__
AttributeError: 'str' object has no attribute '__module__'

Steps to Reproduce

1. Set up a Django project with Celery 5.6.1
2. Install Flower 2.0.1
3. Run Flower: celery -A your_app flower --port=5555
4. Observe the AttributeError in logs

Expected Behavior

Flower should start without errors.

Actual Behavior

An AttributeError is raised because pool_cls is a string instead of a class object when the Django fixup code tries to check if the pool is prefork.

Root Cause Analysis

In celery/fixups/django.py line 220:

is_prefork = "prefork" in self.worker.pool_cls.__module__

The code assumes pool_cls is a class with a __module__ attribute, but at this point in the initialization, pool_cls is still a string (e.g., 'prefork').

Suggested Fix

Check if pool_cls is a string before accessing .__module__:

pool_cls = self.worker.pool_cls
if isinstance(pool_cls, str):
    is_prefork = "prefork" in pool_cls
else:
    is_prefork = "prefork" in pool_cls.__module__

Impact

• Severity: Low (Flower still works despite the error)
• Workaround: None needed - the error is logged but doesn't prevent Flower from functioning