Fix Python – Difference between coroutine and future/task in Python 3.5?


Asked By – knite

Let’s say we have a dummy function:

async def foo(arg):
    result = await some_remote_call(arg)
    return result.upper()

What’s the difference between:

import asyncio    

coros = []
for i in range(5):

loop = asyncio.get_event_loop()


import asyncio

futures = []
for i in range(5):

loop = asyncio.get_event_loop()

Note: The example returns a result, but this isn’t the focus of the question. When return value matters, use gather() instead of wait().

Regardless of return value, I’m looking for clarity on ensure_future(). wait(coros) and wait(futures) both run the coroutines, so when and why should a coroutine be wrapped in ensure_future?

Basically, what’s the Right Way ™ to run a bunch of non-blocking operations using Python 3.5’s async?

For extra credit, what if I want to batch the calls? For example, I need to call some_remote_call(...) 1000 times, but I don’t want to crush the web server/database/etc with 1000 simultaneous connections. This is doable with a thread or process pool, but is there a way to do this with asyncio?

2020 update (Python 3.7+): Don’t use these snippets. Instead use:

import asyncio

async def do_something_async():
    tasks = []
    for i in range(5):
    await asyncio.gather(*tasks)

def do_something():

Also consider using Trio, a robust 3rd party alternative to asyncio.

Now we will see solution for issue: Difference between coroutine and future/task in Python 3.5?


A comment by Vincent linked to, which shows that wait() wraps the coroutines in ensure_future() for you!

In other words, we do need a future, and coroutines will be silently transformed into them.

I’ll update this answer when I find a definitive explanation of how to batch coroutines/futures.

This question is answered By – knite

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0