Python asyncio context

Question

In threading, we have something called "Thread Context", in which we can save some data (state) for accessing in a special thread. In asyncio, I need to save some state in current execution path, so that all consequent coroutines can access it. What is the solution? Note: I know each coroutine function is instantiated for an execution path in asyncio, but for some reason I can not save the state in function properties. (Although this method os not very good anyway)

Answer 1

As of Python 3.7 you can make use of contextvars.ContextVar.

In the example below I declared request_id and set the value in some_outer_coroutine, then accessed it in some_inner_coroutine.

import asyncio import contextvars  # declare context var request_id = contextvars.ContextVar('Id of request.')   async def some_inner_coroutine():     # get value     print('Processed inner coroutine of request: {}'.format(request_id.get()))   async def some_outer_coroutine(req_id):     # set value     request_id.set(req_id)      await some_inner_coroutine()      # get value     print('Processed outer coroutine of request: {}'.format(request_id.get()))   async def main():     tasks = []     for req_id in range(1, 5):         tasks.append(asyncio.create_task(some_outer_coroutine(req_id)))      await asyncio.gather(*tasks)   if __name__ == '__main__':     asyncio.run(main()) 

Output:

Processed inner coroutine of request: 1 Processed outer coroutine of request: 1 Processed inner coroutine of request: 2 Processed outer coroutine of request: 2 Processed inner coroutine of request: 3 Processed outer coroutine of request: 3 Processed inner coroutine of request: 4 Processed outer coroutine of request: 4 

Answer 2

There's also https://github.com/azazel75/metapensiero.asyncio.tasklocal, but you must be aware that tasks are often created internally by libraries and also by asyncio using ensure_future(a_coroutine) and there's no actual way to track these new tasks and initialize their locals (maybe with those of the task that they are created from). (an "hack" whould be setting a loop.set_task_factory() function with something that does the job, hoping that all code uses loop.create_task() to create the tasks, which is not always true...)

Another issue is that if some of your code is executed inside a Future callback Task.current_task() function which is used by both the libraries to select the right copy of locals to serve will always return None...