Celery Bug in celery.contrib.testing.worker.start

The Fix

pip install celery==5.3.0b2

Based on closed celery/celery issue #7726 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -72,6 +72,7 @@ def start_worker(
     test_worker_starting.send(sender=app)
 
+    worker = None
     try:
         with _start_worker_thread(app,

repro.py

app = <Celery celery.tests at 0x7f57bdd6ee50>, concurrency = 1, pool = 'prefork'
loglevel = 'debug', logfile = None, perform_ping_check = True
ping_task_timeout = 10.0, shutdown_timeout = 10.0, kwargs = {}
    @contextmanager
    def start_worker(
        app,  # type: Celery
        concurrency=1,  # type: int
        pool='solo',  # type: str
        loglevel=WORKER_LOGLEVEL,  # type: Union[str, int]
        logfile=None,  # type: str
        perform_ping_check=True,  # type: bool
        ping_task_timeout=10.0,  # type: float
        shutdown_timeout=10.0,  # type: float
        **kwargs  # type: Any
    ):
        # type: (...) -> Iterable
        """Start embedded worker.
    
        Yields:
            celery.app.worker.Worker: worker instance.
        """
        test_worker_starting.send(sender=app)
    
        try:
            with _start_worker_thread(app,
                                      concurrency=concurrency,
                                      pool=pool,
                                      loglevel=loglevel,
                                      logfile=logfile,
                                      perform_ping_check=perform_ping_check,
                                      shutdown_timeout=shutdown_timeout,
                                      **kwargs) as worker:
                if perform_ping_check:
                    from .tasks import ping
                    with allow_join_result():
                        assert ping.delay().get(timeout=ping_task_timeout) == 'pong'
    
                yield worker
        finally:
          test_worker_stopped.send(sender=app, worker=worker)
E           UnboundLocalError: local variable 'worker' referenced before assignment
/usr/local/lib/python3.7/site-packages/celery/contrib/testing/worker.py:91: UnboundLocalError

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install celery==5.3.0b2\nWhen NOT to use: Do not use this fix if the worker's initialization logic changes significantly.\n\n

Why This Fix Works in Production

Trigger: When `_start_worker_thread` fails, the code the `finally` fails as well because `worker` is not defined.
Mechanism: The variable 'worker' was not defined before being referenced in the finally block
Why the fix works: Fixes an issue where a variable used in a finally block may not be instantiated, leading to an UnboundLocalError. (first fixed release: 5.3.0b2).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.7 in real deployments (not just unit tests).
The variable 'worker' was not defined before being referenced in the finally block
Production symptom (often without a traceback): When `_start_worker_thread` fails, the code the `finally` fails as well because `worker` is not defined.

Proof / Evidence

GitHub issue: #7726
Fix PR: https://github.com/celery/celery/pull/7727
First fixed release: 5.3.0b2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.40

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Hey @woutdenolf :wave:, Thank you for opening an issue”

@open-collective-bot · 2022-08-25 · source

Failure Signature (Search String)

When `_start_worker_thread` fails, the code the `finally` fails as well because `worker` is not defined.
ping_task_timeout = 10.0, shutdown_timeout = 10.0, kwargs = {}

Copy-friendly signature

signature.txt

Failure Signature
-----------------
When `_start_worker_thread` fails, the code the `finally` fails as well because `worker` is not defined.
ping_task_timeout = 10.0, shutdown_timeout = 10.0, kwargs = {}

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
When `_start_worker_thread` fails, the code the `finally` fails as well because `worker` is not defined.
ping_task_timeout = 10.0, shutdown_timeout = 10.0, kwargs = {}

Minimal Reproduction

repro.py

app = <Celery celery.tests at 0x7f57bdd6ee50>, concurrency = 1, pool = 'prefork'
loglevel = 'debug', logfile = None, perform_ping_check = True
ping_task_timeout = 10.0, shutdown_timeout = 10.0, kwargs = {}
    @contextmanager
    def start_worker(
        app,  # type: Celery
        concurrency=1,  # type: int
        pool='solo',  # type: str
        loglevel=WORKER_LOGLEVEL,  # type: Union[str, int]
        logfile=None,  # type: str
        perform_ping_check=True,  # type: bool
        ping_task_timeout=10.0,  # type: float
        shutdown_timeout=10.0,  # type: float
        **kwargs  # type: Any
    ):
        # type: (...) -> Iterable
        """Start embedded worker.
    
        Yields:
            celery.app.worker.Worker: worker instance.
        """
        test_worker_starting.send(sender=app)
    
        try:
            with _start_worker_thread(app,
                                      concurrency=concurrency,
                                      pool=pool,
                                      loglevel=loglevel,
                                      logfile=logfile,
                                      perform_ping_check=perform_ping_check,
                                      shutdown_timeout=shutdown_timeout,
                                      **kwargs) as worker:
                if perform_ping_check:
                    from .tasks import ping
                    with allow_join_result():
                        assert ping.delay().get(timeout=ping_task_timeout) == 'pong'
    
                yield worker
        finally:
          test_worker_stopped.send(sender=app, worker=worker)
E           UnboundLocalError: local variable 'worker' referenced before assignment
/usr/local/lib/python3.7/site-packages/celery/contrib/testing/worker.py:91: UnboundLocalError

Environment

Python: 3.7

What Broke

UnboundLocalError occurs when the worker fails to start, causing confusion during debugging.

Why It Broke

The variable 'worker' was not defined before being referenced in the finally block

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install celery==5.3.0b2

When NOT to use: Do not use this fix if the worker's initialization logic changes significantly.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/celery/celery/pull/7727

First fixed release: 5.3.0b2

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if the worker's initialization logic changes significantly.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

Version Compatibility Table

Version	Status
5.3.0b2	Fixed

Related Issues

No related fixes found.

Cluster: celery:configuration-error Celery hub Celery best practices All hubs All clusters

Related clusters: Data consistency Duplicates Duplicate processing

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Celery Bug in celery.contrib.testing.worker.start_worker (Fix)