Python asyncio add_done_callback with async def

Question

I have 2 functions: the first, def_a , is an asynchronous function, and the second is def_b , which is a regular function and is called with the result of def_a as a callback using the add_done_callback function.

My code is as follows:

 import asyncio def def_b(result): next_number = result.result() # some work on the next_number print(next_number + 1) async def def_a(number): await some_async_work(number) return number + 1 loop = asyncio.get_event_loop() task = asyncio.ensure_future(def_a(1)) task.add_done_callback(def_b) response = loop.run_until_complete(task) loop.close() 

And it works great.

The problem started when the second def_b function became asynchronous. Now it looks like this:

 async def def_b(result): next_number = result.result() # some asynchronous work on the next_number print(next_number + 1) 

But now I can not provide it to the add_done_callback function, because it is not a regular function.

My question is: is this possible and how can I provide def_b to the def_b function if def_b is asynchronous?

Answer 1

add_done_callback is considered a "low level" interface. When working with coroutines, you can link them in different ways, for example:

 import asyncio async def my_callback(result): print("my_callback got:", result) return "My return value is ignored" async def coro(number): await asyncio.sleep(number) return number + 1 async def add_success_callback(fut, callback): result = await fut await callback(result) return result loop = asyncio.get_event_loop() task = asyncio.ensure_future(coro(1)) task = add_success_callback(task, my_callback) response = loop.run_until_complete(task) print("response:", response) loop.close() 

Keep in mind that add_done_callback will still call a callback if your future throws an exception (but calling result.result() will raise it).

Answer 2

This only works for one future job, if you have several asynchronous tasks, they will block each other, it is better to use asyncio.as_comleted () to iterate the future list:

 import asyncio async def __after_done_callback(future_result): # await for something... pass async def __future_job(number): await some_async_work(number) return number + 1 loop = asyncio.get_event_loop() tasks = [asyncio.ensure_future(__future_job(x)) for x in range(100)] # create 100 future jobs for f in asyncio.as_completed(tasks): result = await f await __after_done_callback(result) loop.close()