Safely add a key to a dict only if it does not already exist?

[Vito De Tullio]

Chris Rebert wrote:

>> How can I add a key in a thread-safe manner?
> I'm not entirely sure, but have you investigated dict.setdefault() ?

but how setdefault makes sense in this context? It's used to set a default 
value when you try to retrieve an element from the dict, not when you try to 
set a new one ...

-- 
ZeD