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

[Ian Kelly]

On Fri, Jul 6, 2018 at 11:56 AM INADA Naoki <songofacandy at gmail.com> wrote:
>
> D.setdefault('c', None)

Not guaranteed to be atomic.

I think the only safe way to do it is with a Lock.