Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Chris Angelico <rosuav@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: cross platform alternative for signal.SIGALRM?
Date: Thu, 12 Nov 2015 18:37:02 +1100
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On Thu, Nov 12, 2015 at 5:43 PM, Christian Gollwitzer <auriocus@gmx.de> wrote:
> My understanding of async is that it creates an event loop. In which case
> the loop has no chance to run within a block of code that computes anything,
> is that correct?

This is correct. At its simplest, asynchronous code is an abstraction
over the select() call, which basically says "Hey system, tell me when
(a) I can read from here, (b) I can write to here, or (c) I've been
waiting this long". The most common use is sockets; a web server has
its main listening socket (it becomes readable when someone connects),
any clients that haven't finished sending their requests yet (they
become readable when more data arrives), any clients that you're still
sending to (they become writeable when there's room in their output
buffers), and maybe some sort of periodic checks ("every hour, do
maintenance"). Whenever you finish a bit of processing (reading from a
client, sending to a client, whatever), you return to the "event
loop", which in this case would be select().

An async library makes all this look a lot cleaner in your code, but
ultimately, it's not preemptive. You still have to make sure the
processing doesn't take too long.

ChrisA