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

Chris Rebert clp2 at
Sat Jan 19 05:15:26 CET 2013

On Friday, January 18, 2013, Steven D'Aprano wrote:

> I wish to add a key to a dict only if it doesn't already exist, but do it
> in a thread-safe manner.
> The naive code is:
> if key not in dict:
>     dict[key] = value
> but of course there is a race condition there: it is possible that

another thread may have added the same key between the check and the
> store.
> How can I add a key in a thread-safe manner?

I'm not entirely sure, but have you investigated dict.setdefault() ?

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