[Python-Dev] Importance of "async" keyword

[Steve Dower]

Ethan Furman wrote:
> On 06/26/2015 08:47 AM, Steve Dower wrote:
>> On 06/26/2015 06:48 AM, Sven R. Kunze wrote:
>>
>>> def business():
>>> return complex_calc(5)
>>>
>>> def business_new()
>>> return await complex_calc(10)
> 
>> Assuming "async def business_new" (to avoid the syntax error), there's no
>> difference between those functions or the one they're calling:
>>
>> * "complex_calc" returns an awaitable object that, after you've awaited it,
>> will result in an int.
>> * "business" returns the return value of "complex_calc", which is an awaitable
>> object that, after you've awaited it, will result in an int.
>> * "business_new" returns an awaitable object that, after you've awaited it,
>> will result in an int.
>>
>> In all three of these cases, the result is the same. The fact that the
>> awaitable object returned from any of them is implemented by a coroutine isn't
>> important (in the same way that an iterable object may be implemented by a
>> generator, but it's really irrelevant).
> 
> What? Shouldn't 'business_new' return the int? It did await, after all.

I assumed "async def business_new()", rather than some imaginary "await in a non-async function will block because I love to create deadlocks in my code" feature.

Note that blocking prevents *all* coroutines from making progress, unlike threading. When you await all the way to an event loop, it defers the rest of the coroutine until a signal (via a callback) is raised and continues running other coroutines.

Cheers,
Steve