Thread-safe way to add a key to a dict only if it isn't already there?

[Chris Angelico]

On Mon, Jul 9, 2018 at 2:11 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
> MRAB <python at mrabarnett.plus.com>:
>> On 2018-07-08 14:38, Steven D'Aprano wrote:
>>> On Sun, 08 Jul 2018 14:11:58 +0300, Marko Rauhamaa wrote:
>>>
>> [snip]
>>>> More importantly, this loop may never finish:
>>>>
>>>>     # Initially
>>>>     quit = False
>>>>
>>>>     # Thread 1
>>>>     global quit
>>>>     while not quit:
>>>>         time.sleep(1)
>>>>
>>>>     # Thread 2
>>>>     global quit
>>>>     quit = True
>>>
>>> Assuming that thread 2 actually runs *at some point*, I don't see how
>>> that can't terminate. Neither thread sets quit to False, so provided
>>> thread 2 runs at all, it has to terminate.
>>>
>> [snip]
>>
>> The compiler could look at the code for thread 1 and see that 'quit' is
>> never assigned to, meaning that it could be "optimised" to:
>>
>>     global quit
>>     if not quit:
>>         while True:
>>             time.sleep(1)
>>
>> In C you'd declare 'quit' as 'volatile' to tell the compiler that it
>> could change unexpectedly, so don't make that assumption.
>
> C is an even tougher case. Even if the compiler kept on checking a
> volatile value, the CPU might never propagate the cache content to the
> other core. You'd need a memory barrier. In Java, "volatile" effectively
> creates a memory barrier, but in C (and C++) it does not. In C you need
> something like a mutex to see the effects of other threads running.
>
> (BTW, I think that's a terrible thing for the C standards committee to
> specify.)

None of this has any impact on Python whatsoever.

ChrisA