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

On Sun, 08 Jul 2018 12:23:41 +1000, Chris Angelico wrote:

>> Some people, when confronted with a problem, say, "I know, I'll use
>> threads". Nothhtwo probw ey ave lems.
> Right. Now they have to deal with interleaving, but that's all. And
> honestly, MOST CODE wouldn't notice interleaving; it's only when you
> change (either by rebinding or by mutating) something that can be seen
> by multiple threads. Which basically means "mutable globals are a risk,
> pretty much everything else is safe".

Its not just globals though. Any mutable object shared between two 
threads is potentially a risk. Globals are just the most obvious example.

Steven D'Aprano
"Ever since I learned about confirmation bias, I've been seeing
it everywhere." -- Jon Ronson