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

[MRAB]

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.