Thread safety issue (I think) with defaultdict

[Steve D'Aprano]

On Fri, 3 Nov 2017 07:24 am, Chris Angelico wrote:

> On Fri, Nov 3, 2017 at 3:27 AM, Israel Brewster <israel at ravnalaska.net>
> wrote:
>>
>> Actually, that saying is about regular expressions, not threads :-) . In
>> the end, threads are as good a way as handling concurrency as any other,
>> and simpler than many. They have their drawbacks, of course, mainly in the
>> area of overhead, and of course only multiprocessing can *really* take
>> advantage of multiple cores/CPU's on a machine, but unlike regular
>> expressions, threads aren't ugly or complicated. Only the details of
>> dealing with concurrency make things complicated, and you'll have to deal
>> with that in *any* concurrency model.
>>
> 
> Thank you. I've had this argument with many people, smart people (like
> Steven), people who haven't grokked that all concurrency has costs -

Of course I grok that all concurrency has costs. Apart from comparatively rare
cases of "embarrassingly parallel" algorithms, any form of concurrent or
parallel processing is significantly harder than sequential code.


> that threads aren't magically more dangerous than other options.

There's nothing magical about it.

Threads are very much UNMAGICALLY more dangerous than other options because
they combine:

- shared data; and

- non-deterministic task switching.

Having both together is clearly more dangerous than only one or the other:

- async: shared data, but fully deterministic task switching;

- multiprocessing: non-deterministic task switching, but by default
  fully isolated data.




-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.