python asyncio add_done_callback with async def

Question

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

My code looks like this:

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's work perfectly.

The problem began when also the second function, def_b, 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's not a regular function.

My question is- Is it possible and how can I provide def_b to the add_done_callback function if def_b is asynchronous?

Answer 1

add_done_callback is considered a "low level" interface. When working with coroutines, you can chain them in many 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 add_done_callback will still call the callback if your future raises an exception (but calling result.result() will raise it).