[Tutor] What exactly does "await" do? (David)

[Mats Wichmann]

On 2/13/23 21:40, Alphonsus Okoye wrote:
>> ---------- Forwarded message ----------

> Good day Mr David and to everyone here. From what you said, the coroutine
> can be executed just like a function; the execution of the main function is
> suspended till the inner function has finished executing. What is then the
> main purpose of a coroutine?

Wouldn't express it quite this way.  In an async program, you have a 
main part of the program that keeps running - usually called an event 
loop.  It decides if there's any work that can be done, in the form of 
resuming things that are now ready.  The await keyword is you telling 
Python that this is a point where it's okay to stash the current state 
and go off and do other work.  So if you fired off a request to a 
webserver, which is "slow" in terms of computer time, and you flag with 
await that it's okay not to stall execution until it responds, then the 
main loop picks back up and figures out if there's other work that can 
be done.  At some point later, the main loop will spot that the thing 
being awaited is done, and it can restart execution with that saved 
context. The restarted code then has to give up control again so the 
main loop can continue - either by returning from the function, or 
another await. And so on.

A coroutine is just a function that is able to be suspended/restarted, 
which you flag to Python by using the async keyword - you have to have 
indicated that it's a coroutine to be allowed to use await in it. The 
async/await syntax was the approach chosen to retrofit async 
capabilities into an already mature language without breaking the 
behavior of tons of existing code which does not use async principles 
(and doesn't need to).