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

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):
    coros.append(foo(i))

loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait(coros))

And:

import asyncio

futures = []
for i in range(5):
    futures.append(asyncio.ensure_future(foo(i)))

loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait(futures))

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):
        tasks.append(asyncio.create_task(foo(i)))
    await asyncio.gather(*tasks)

def do_something():
    asyncio.run(do_something_async)

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