Get more context for task execution

When declaring a task, you can request for more context to be passed to the task function:

@app.task(..., pass_context=True)
def mytask(context: procrastinate.JobContext, ...):
    ...

This serves multiple purposes. The first one is for introspection, logs, etc. The JobContext object contains all sort of useful information about the worker that executes the job.

The other useful feature is that you can pass arbitrary context elements using App.run_worker() (or App.run_worker_async()) and its additional_context argument. In this case the context the task function receives will have an additional_context attribute corresponding to the elements that were passed:

@app.task(pass_context=True)
def mytask(context: procrastinate.JobContext):
    http_session = context.additional_context["http_session"]
    return await http_session.get("www.example.com")

async with AsyncSession() as http_session:
    await app.run_worker_async(additional_context={"http_session": http_session})
    ...

This feature is not supposed to be used for passing data from a task to a future task. In order to deter this behavior, Procrastinate will not keep modifications made to the additional_context dict after the worker has started.

That being said, the values kept in this dict are not processed by Procrastinate. Any task mutating a value inside this dict will impact what all the concurrent and following tasks will read.