In celery, what is the appropriate way to pass contextual metadata from sender process to worker when a task is enqueued?

Question

When any celery task is enqueued I want to add contextual metadata the worker will be able to use.

The following code example works but I would like to have an appropriate celery-style solution.

from celery.signals import before_task_publish, task_prerun

@before_task_publish.connect
def receiver_before_task_publish(sender=None, headers=None, body=None, **kwargs):
    task_kwags = body[1]
    metadata = {"foo": "bar"}
    task_kwags['__metadata__'] = metadata

@task_prerun.connect
def receiver_task_pre_run(task_id, task, *args, **kwargs):
    metadata = kwargs['kwargs'].pop('__metadata__', {})
    # metadata == {"foo": "bar"}

Answer 1

When a task starts in the worker the content of before_task_publish's header is in the **kwargs of push_request.

celery/app/tasks.py:1000

    def push_request(self, *args, **kwargs):
        self.request_stack.push(Context(*args, **kwargs))

Something nice is done in the constructor of Context. self.__dict__.update() means we can access the values as Context(metadata={'foo': 'bar'}).metadata

celery/app/tasks.py:99

class Context(object)
# ...
    def __init__(self, *args, **kwargs):
        self.update(*args, **kwargs)

    def update(self, *args, **kwargs):
        return self.__dict__.update(*args, **kwargs)

The task context is accessible from Task's request property.

celery/app/tasks.py:1019

class Task(object):
# ...
    def _get_request(self):
        """Get current request object."""
        req = self.request_stack.top
        if req is None:
            # task was not called, but some may still expect a request
            # to be there, perhaps that should be deprecated.
            if self._default_request is None:
                self._default_request = Context()
            return self._default_request
        return req
    request = property(_get_request)

Which means the final solution is simply this:

from celery.signals import before_task_publish, task_prerun

@before_task_publish.connect
def receiver_before_task_publish(sender=None, headers=None, body=None, **kwargs):
    metadata = {"foo": "bar"}
    headers['__metadata__'] = metadata

@task_prerun.connect
def receiver_task_pre_run(task_id, task, *args, **kwargs):
    metadata = getattr(task.request, '__metadata__', {}) 
    # metadata == {"foo": "bar"}

Note: task.request.__metadata__ would also work but it fails if a task was enqueued before the signals are integrated. Safer this way.