Re: 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.
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